1 Introduction
The printed electronics technology has drawn increasing attention in previous decades as a promising deposition technology [1,2]; some examples of such technologies include spray coating [3,4], inkjet printing [5,6], and screen printing [7,8]. The aforementioned solution-based deposition methods allow the low-cost preparation of large-area, flexible, or patterned devices. In this technology, one of the most important factors is the usage of functional inks containing a well-dispersed active material, which demand the vigorous development of materials science [9–12].
Two-dimensional (2D) materials with unique structural properties exhibit superior performance with respect to various applications such as energy storage [13,14], optoelectronics [15,16], and catalysis [17,18]. Moreover, when combined with the printed electronics technology, 2D materials can easily construct functional devices, including transistors [19,20], supercapacitors [21], and light-emitting diodes (LEDs) [22], in a fast and simple manner. Thus, studies on inks containing 2D materials should not be neglected owing to the promising demonstration of the liquid-phase exfoliation of 2D materials such as graphene [23], MXenes [24], transition metal dichalcogenides (TMDs) [25], and transition metal oxides (TMOs) [26]. Typically, 2D materials can be stabilized in organic solvents, the surface energies of which match with those of the 2D materials [23]. However, organic solvents are often toxic and costly. Water is considered to be an ideal solvent for preparing ink because it is environmentally friendly, inexpensive, and abundant. Generally, 2D materials require the assistance of surfactants to disperse well in water [27]. However, the surfactants on their surface are difficult to remove, which probably limits the performance of the devices. Therefore, exploring an efficient methodology to directly disperse 2D materials in water is essential for the development of 2D material inks.
Tungsten oxide (WO3) having several outstanding properties is a promising material for electrochromic [28,29], sensing [30], and catalytic [31] applications. Nevertheless, few studies have investigated 2D WO3-based aqueous inks. Liang et al. prepared an aqueous dispersion of WO3·H2O nanosheets through the ultrasonic exfoliation of bulk WO3·2H2O in N,N-dimethylformamide (DMF), which considerably increased the complexity of the preparation process [32]. Herein, we report a novel strategy for improving the dispersity of the Na2W4O13 nanosheets in pure water via the Li-ion exchange method. The partially Li-ion-exchanged Na2W4O13 (LixNa2−xW4O13) nanosheets show a highly stable dispersity in water with a zeta potential of −55 mV. Moreover, this aqueous ink can be sprayed on various substrates to obtain a uniform LixNa2−xW4O13 nanosheet film, exhibiting an excellent electrochromic performance. The electrochromic device assembled using a Prussian white (PW) film and a sprayed LixNa2−xW4O13 nanosheet film exhibits large optical modulation, fast switching response, and considerable cyclic stability.
2 Experimental section
Synthesis of Na2W4O13 nanosheets: Na2W4O13 nano-sheets were synthesized using the molten salt method [14]. 5 g of sodium nitrate (NaNO3) was added to a crucible and transferred to a muffle furnace at a temperature of 350°C. When NaNO3 was completely melted, 0.2 g of ammonium tungstate hydrate (H40N10O41W12·xH2O) powder was added into molten NaNO3 for approximately 1 min. Then, the crucible was removed from the muffle furnace and cooled to room temperature. Finally, the Na2W4O13 nanosheets were collected via vacuum filtration after soaking and washing in deionized (DI) water and drying at 70°C.
Preparation of the LixNa2−xW4O13 nanosheet aqueous ink: LixNa2−xW4O13 nanosheets were prepared in two steps. Typically, 40 mg of the Na2W4O13 nanosheets was added to 40 mL of the 1 mol/L lithium sulfate (Li2SO4) aqueous solution under stirring for 3–7 days at room temperature. After vacuum filtering the suspension and washing it several times using DI water, LixNa2−xW4O13 nanosheets were obtained. Then, this obtained product was dispersed in DI water by probe sonication and centrifuged at 2000 r/min to prepare the LixNa2−xW4O13 nanosheet aqueous ink.
Fabrication of the LixNa2−xW4O13 nanosheet film: The LixNa2−xW4O13 nanosheet film was prepared on fluorine-doped tin oxide (FTO) glass and flexible indium tin oxide (ITO) substrate by spray coating. The LixNa2−xW4O13 nanosheet ink was sprayed on the substrate maintained at a temperature of 100°C.
Fabrication of the PW film: The Prussian blue (PB) film was electrodeposited on FTO glass via galvanostatic deposition using a 0.01 mol/L K3[Fe(CN)]6, 0.01 mol/L FeCl3, and 0.05 mol/L KCl aqueous solution as the electrolyte [33]. Electrochemical deposition was performed under a constant current density of −50 µA/cm2 for 120 s using clean FTO glass or flexible ITO substrates as the working electrode, a carbon rod as the counter electrode, and Ag/AgCl as the reference electrode. The PW film was obtained from the PB film from a 1 mol/L LiClO4/propylene carbonate (PC) solution.
Assembly of the electrochromic device: The electrochromic device was assembled using the sprayed LixNa2−xW4O13 nanosheet film as the electrochromic layer, PW film as the ion storage layer, and a 1 mol/L LiClO4/PC solution as the electrolyte. These two functional layers were separated by a narrow tape at the edges and sealed using epoxy glue.
Characterization: The crystal structure of the materials was determined by X-ray diffraction (XRD) using Cu Kα radiation (l = 1.5418 Å) (X’Pert Pro, PANalytical). The zeta potential was measured using a Zetasizer (Nano ZSP, Malvern Instruments Limited, UK). Field-emission scanning electron microscopy (FE-SEM, FEI Nova 450 Nano) and transmission electron microscopy (Titan G2 60-300) were employed to explore the structure and morphology of the materials. The chemical composition and oxidized state of materials were studied using X-ray photoelectron spectroscopy (XPS, ESCALab 250). The atomic ratio of the elements and concentration of the dispersions were measured using inductively coupled plasma optical emission spectrometry (ICP–OES, Optima 4300DV). The optical spectrum and in situ transmittance response were obtained using a UV spectrophotometer (Shimadzu UV-3600Plus), and the electrochemical tests of electrochromic performance were performed using CHI660D.
Density functional theory calculations: All the density functional theory calculations were performed using the projected augmented wave method and Perdew–Burke–Ernzerhof (PBE) functional within the generalized gradient approximation, as implemented in the Vienna ab initio Simulation Package (VASP) [34–37]. The energy cutoff for plane wave expansion was set to 650 eV. All the surfaces were simulated using the slab model. And the same terminations were modelled at the top and bottom of the slab. The atoms in the middle five, two, three, and three layers of (001), (100), (011), and () facets were fixed at their optimal bulk positions, respectively, whereas the remaining atoms were fully relaxed until the force on each atom was less than 0.02 eV/Å. An implicit solvation model was employed to simulate the water solvent in experiments[38]. The surface energies and interfacial energies were calculated using
where Eslab is the energy of the entire slab, Ebulk is the energy of the corresponding bulk phase, A is the surface area of the slab, is the number of oxygen atoms in the system, is half the total energy of the oxygen molecule, is the number of metal atoms in the system, and is the energy per atom of the bulk metal.
3 Results and discussion
3.1 Li-ion-exchanged Na2W4O13 nanosheets
The Li-ion exchange process is illustrated in Fig. 1(a). The Na2W4O13 nanosheets (Fig. S1) were synthesized using the molten salt method, as described in a previous study [14]. Li-ion exchange could occur with the addition of Na2W4O13 nanosheets into the Li2SO4 solution, which was under stirring for several days. The Li-ion-exchanged sample was collected by vacuum filtration after being extensively soaked and washed using DI water, which was followed by drying at 70°C. The dispersity of the Na2W4O13 nanosheets in DI water significantly improved after the Li-ion exchange process. Figure 1(b) shows the optical image of the ink of pristine Na2W4O13 and LixNa2−xW4O13 having the same concentration of 1 mg/mL. The dispersion of LixNa2−xW4O13 is more transparent with a strong Tyndall effect owing to the nanosize of the material, which suggests better dispersibility than the pristine Na2W4O13. Furthermore, the sedimentation curves of suspension when the same initial concentration of 2 mg/mL is considered are shown in Fig. 1(c). Figure 1(c) shows that the concentration of the pristine Na2W4O13 nanosheets decreased to become less than 0.8 mg/mL after three days, whereas the concentration of the LixNa2−xW4O13 nanosheets became greater than 1.2 mg/mL after only two weeks. This improved dispersity of the LixNa2−xW4O13 nanosheets in DI water was further confirmed based on the enhanced zeta potential (Fig. 1(d)). According to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory on colloid stability, the dispersed nanosheets, which can be considered to be nanoparticles, do not undergo aggregation because of the sufficient electrostatic repulsive force caused by the electric double layer around the nanosheets [39,40]. The resultant repulsion is generally characterized by the zeta potential. As shown in Fig. 1(d), with an increase in the Li-ion exchange time, the zeta potential of the modified nanosheets decreased from −40 to −55 mV, indicating the improved dispersity in water. The atomic ratio of sodium and tungsten in these samples gradually decreased from 0.46 to 0.19, which was determined by following the Li-ion exchange process based on the inductively coupled plasma optical emission spectroscopy (ICP–OES) analysis. Moreover, the total content of alkali metal ions (Li and Na) in these samples remained the same as that of the initial Na2W4O13 (Fig. S2), which indicated that the exchanged Li ions in the sample improve the dispersity.
Fig.1 (a) Schematic of the Li-ion exchange process for the Na2W4O13 nanosheets. (b) Photographs of the pristine and LixNa2−xW4O13 nanosheets dispersed in DI water with a concentration of 1 mg/mL (inset: demonstration of the Tyndall effect with respect to the dispersion of Li-ion-exchanged nanosheets). (c) Sedimentation curves of the pristine LixNa2−xW4O13 nanosheets dispersed in DI water with an initial concentration of 2 mg/mL. (d) Dependence of the atomic ratio of sodium compared with that of tungsten (black) and zeta potential in DI water (red) of the LixNa2−xW4O13 nanosheets on the Li-ion exchange time. (e) XRD patterns of the pristine LixNa2−xW4O13 nanosheets. (f) TEM image of the LixNa2−xW4O13 nanosheets. (g) HRTEM and SAED images of the LixNa2−xW4O13 nanosheets |
XRD was employed to identify the crystal structure of Na2W4O13 during the Li-ion exchange process (Fig. 1(e)). There is no difference between the XRD patterns of pristine Na2W4O13 and LixNa2−xW4O13, and the diffraction peaks are consistent with those of Na2W4O13 (JCPDS: 21-1167). The morphology of these samples was explored by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figures 1(f) and S3 show that LixNa2−xW4O13 retained its original 2D morphology. Moreover, the high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SEAD) images show the regular atomic arrangement and good crystallinity of the LixNa2−xW4O13 nanosheets (Fig. 1(g)). XPS was performed to investigate the oxidized state of the LixNa2−xW4O13 nanosheets. Figure S4 shows the high-resolution XPS spectrum of the W4f peak. The spin-orbit doublets in the spectrum correspond to the W4f 7/2 and W4f 5/2 peaks, which are located at 35.6 and 37.8 eV, respectively, indicating that W is at the highest oxidized state (W6+) [41].
3.2 Density functional theory calculations
Density functional theory (DFT) calculations were used to elucidate the improved dispersity of the Li-ion exchanged Na2W4O13 nanosheets in DI water. First, the formation energies of the stable LixNa2−xW4O13 structures were determined using the convex hull method, as implemented in the Clusters Approach to Statistical Mechanics (CASM) [42,43] package (Fig. 2(a)). Specifically, a total of 8688 symmetrically distinct LixNa2−xW4O13 structures were considered in our calculation, among which the formation energies of 175 configurations (denoted as blue solid circles in Fig. 2(a)) were obtained via DFT calculations, whereas those of the remaining 8513 configurations (denoted as black empty circles in Fig. 2(a)) were predicted using the cluster expansion (CE) method. Thus, only one stable Li-ion exchanged structure having a composition of Li1.333Na0.666W4O13 was predicted, as illustrated in Fig. 2(a). However, the lowest formation energies of different Li-ion exchanged LixNa2−xW4O13 configurations at each Li concentration were less than zero, indicating that the exchange of Na ions with Li ions in the Na2W4O13 nanosheets was energetically favorable, as demonstrated experimentally.
Fig.2 (a) Formation energies per formula unit versus the fractional lithium concentration in LixNa2−xW4O13. The convex hull (tie line) is constructed by jointing the stable structures obtained by the searches. (b) Interfacial energies of four low-index facets of Na2W4O13 and Li2W4O13 with favorable terminations in water |
The good dispersity of the as-prepared samples in water mainly depends on the stable interface of the exposed surfaces of nanoparticles and water. Thus, the interfacial energy can be used to evaluate the dispersity of the nanosheets in water; a low interfacial energy indicates the good dispersity of nanosheets in water and vice versa. Thus, we investigated the interfacial features of pristine Na2W4O13 and Li2W4O13 in water and compared their corresponding interfacial energies with respect to water by employing the implicit solvation scheme. Four low-index facets of pristine Na2W4O13, i.e., (001), (100), (011), and (), were considered to completely evaluate the stability of the interface. The favorable termination of these facets was determined by the surface energies in vacuum, as shown in Fig. S5 and Table S1. Based on the favorable termination of four facets in case of pristine Na2W4O13, we obtained four low-index facets of Li2W4O13 having the same terminations (Fig. S6). In case of all the considered facets of Na2W4O13 and Li2W4O13, the outermost layers comprised alkali metal ions, which would be directly exposed to water. With the introduction of water, the interfacial energies of the Li2W4O13 surfaces were less than those of the Na2W4O13 surfaces with respect to the same low-index facets (Fig. 2(b) and Table S2). Because identical facets and relevant terminations were considered for Li2W4O13 and Na2W4O13, the interfaces of Li2W4O13 may be stabilized by the high affinity of Li ions with respect to water. This is particularly important in case of Na2W4O13 owing to the exchange of Li ion with Na ion, which results in the excellent dispersity of the LixNa2−xW4O13 nanosheets in water, as observed experimentally.
3.3 Sprayed LixNa2−xW4O13 nanosheet film
Because the LixNa2−xW4O13 nanosheets can disperse well in DI water, the LixNa2−xW4O13 dispersion can be used as a functional ink in case of the printed electronics technology. A uniform LixNa2−xW4O13 nanosheet film coating was obtained on the substrate when this ink was directly sprayed on a hot substrate (Fig. 3(a)). As shown in Fig. 3(b), the LixNa2−xW4O13 nanosheets can be clearly observed in the top-view SEM image of the film on the FTO glass. In addition, the cross-section view SEM image (Fig. 3(c)) reveals that the LixNa2−xW4O13 nanosheets are compactly stacked on the FTO glass and form a film with a relatively uniform thickness of 350 nm. The as-obtained film exhibits an extremely high optical transmittance of ~90% (the transmittance of the FTO glass is used as the baseline). In addition, the film has strong adhesion to substrate; even the tape fails to damage the film.
Fig.3 (a) Schematic of the spray coating process for the LixNa2−xW4O13 nanosheet ink. (b) Top-view SEM image of the sprayed LixNa2−xW4O13 nanosheet film. (c) Cross-sectional view SEM image of the sprayed LixNa2−xW4O13 nanosheet film. (d) Transmittance spectra of the LixNa2−xW4O13 nanosheet film in the visible range (inset: adhesion). (e) Transmittance spectrum of the LixNa2−xW4O13 nanosheet film under different potentials. (f) In situ optical switching response of the LixNa2−xW4O13 nanosheet film at 550 nm. (g) In situ variation of the optical density (DOD) at 550 nm versus the charge density for the LixNa2−xW4O13 nanosheet film |
WO3 is a classic electrochromic material because of its large optical tunability in the visible range and excellent cyclic stability [28,44]. The LixNa2−xW4O13 nanosheets assembled into a highly transparent film can serve as a promising electrode for electrochromic devices. The electrochromic performance of the sprayed LixNa2−x W4O13 nanosheet film was evaluated in a three-terminal electrochemical cell, where the LixNa2−xW4O13 nanosheet film on FTO was considered to be the working electrode, a Pt plate was considered to be the counter electrode, Ag/AgCl was considered to be the reference electrode, and 1 mol/L LiClO4/PC was considered to be the electrolyte. As shown in Fig. 3(e), the transmittance spectra of the LixNa2−xW4O13 nanosheet film in the visible range were obtained with respect to the applied potentials; the transmittance of the FTO glass in an electrolyte is considered to be the baseline. With increasing negative potential, the color of the film became deeper blue. The contract at 700 nm is approximately 45% under a potential of −1.2 V; the film bleached to denote initial transmittance under a potential of 0.3 V (Fig. S7). The colored state of the sprayed film was uniform, indicating the uniform distribution of the LixNa2−xW4O13 nanosheets.
The switching response, (τa/τb) which is a key parameter with respect to an electrochromic material, is defined as the time required change the total optical modulation by 90% during the coloration or bleaching processes. Because human eyes are most sensitive to a wavelength of approximately 550 nm, the in situ transmittance response was measured at 550 nm. The LixNa2−xW4O13 nanosheet film exhibited a fast switching response of 10.1/1.6 s for the coloration/bleaching process (Fig. 3(f)). The fast switching response benefits from the reduced ion diffusion paths and the increased Li-ion coefficient because of the morphology of nanosheets, which possess a large specific surface area and have more diffusible channels [45,46]. The asymmetric response result of the conductivity variation of the LixNa2−xW4O13 film during the electrochromic process can be obtained from the current curve (Fig. S8). In general, tungsten oxide is more conductive in the colored state (reduced state) accompanied by Li-ion intercalation [45,47].
Another crucial parameter for evaluating the electrochromic performance is the coloration efficiency (CE), which is defined as the change in optical density (DOD) per unit charge (DQ) inserted or extracted from the electrochromic material and can be calculated as follows:
where Tb and Tc refer to the transmittances of the electrochromic material in bleached and colored states, respectively. The relation of DOD to the charge density at 550 nm is shown in Fig. 3(g). The CE value for the spayed LixNa2−xW4O13 nanosheet film calculated based on the slope of the curve is 25.2 cm2/C.
3.4 Assembly of electrochromic device
Typically, electrochromic devices comprise an electrochromic layer, an ion storage layer, an electrolyte layer, and a conductive layer. Here, we use the LixNa2−xW4O13 nanosheet film as the electrochromic layer and PW as the ion storage layer to assemble a complementary electrochromic device (Fig. 4(a)). The electrolyte used for this device is 1 mol/L LiClO4 in the PC solution. Moreover, the PW film can serve as an anodic coloring electrochromic material, and the reversible switch between transparency (PW, reduced state) and blue color (PB, oxidized state) can considerably improve the electrochromic performance of the device. The PW film on the FTO substrate was fabricated by reducing the galvanostatically deposited PB film (Fig. S9). The modulation of transmittance spectra of the electrochromic device (2 cm × 2.5 cm) in the visible range was evaluated with respect to the applied voltages (Fig. 4(d)), where the transmittance of the two FTO glasses sandwiching the electrolyte was considered to be the baseline. The electrochromic device becomes dark blue at a voltage of −2 V (Fig. 4(c)), and the contract at 700 nm reaches ~75%. The bleached state of the electrochromic device can be observed at a voltage of 1 V (Fig. 4(b)), the transmittance of which reaches almost 90% in the visible range. In this case, the switching response and coloration efficiency are superior to those of the sprayed LixNa2−xW4O13 film-based single electrochromic electrode. The coloration time and bleaching time of this electrochromic device are 2 and 1.4 s, respectively (Fig. 4(f), Video S1). The plot of DOD variation at 550 nm versus the considered charge density is shown in Fig. 4(g), which shows a coloration efficiency of 67.2 cm2/C. The cycling stability of this device was tested by the in situ transmittance response at cyclic bias voltages of −2 and 1 V. As shown in Fig. 4(e), the contact at 550 nm of the electrochromic device is maintained at ~40% for more than 20000 s (over 1000 cycles), indicating that the electrochemical performance of the device is extremely stable. Figure S10 shows the detailed switching response of the device during the first 10 cycles and after 1000 cycles.
Fig.4 (a) Schematic of the complementary electrochromic device. (b) Bleaching state of the electrochromic device under a voltage of 1 V. (c) Coloration state of the electrochromic device under a voltage of −2 V. (d) Transmittance spectrum of the electrochromic device under different voltages. (e) In situ transmittance at 550 nm for the measurement of cyclic stability. (f) In situ optical switching response of the electrochromic device at 550 nm. (g) In situ variation of the optical density (DOD) at 550 nm versus the charge density for the electrochromic device |
The solution-based deposition technology shows unique advantages with respect to the manufacturing of large-scale and flexible devices without requiring extremely harsh vacuum and gas atmospheres or expensive equipment. The LixNa2−xW4O13 nanosheet-based ink can be easily scaled up using the facile molten salt method and the Li-ion exchange strategy. This approach exhibits considerable potential for the preparation of large-size devices. Figure S11 shows rigid and flexible electrodes having sizes of 10 cm × 10 cm and 3 cm × 4 cm, respectively, which were fabricated by spraying this ink on the FTO glass and the flexible ITO substrates. A large-area electrochromic smart window was constructed based on these electrodes. As shown in Figs. 5(a) and 5(b), a smart window (9 cm × 10 cm) can reversibly switch between dark blue and transparency based on the coloration and bleaching responses (Video S2). Figure S12(a) shows the dual-band electrochromic modulation [48–50] of the prototype device. Under a voltage of 1 V, the electrochromic device reaches a bright mode with high transmittance in the NIR and VIS ranges. However, the transmittance of the device rapidly decreases to zero in the band of more than 2000 nm owing to the effect of the FTO substrate. Under a voltage of -0.5 V, the electrochromic device reaches the cool mode, which blocks approximately 60% of NIR and ensures high transmittance in the VIS range. When a voltage of −2 V is applied, the VIS transmittance decreases considerably and the device reaches the dark mode (Fig. S12(b)). These results suggest that this smart window effectively modulates light transmission, which probably reduces the energy consumption of a building for refrigeration purpose. In addition, a flexible electrochromic device with a size of 3 cm × 3.5 cm exhibits a facile colored response during the bending state (Fig. S13). These results demonstrate the possibility of using this ink for practical applications as smart windows and flexible displays.
4 Conclusions
In summary, we successfully prepare a stable LixNa2−xW4O13 nanosheet aqueous ink using the Li-ion exchange strategy. By exploiting the spray coating technology, the LixNa2−xW4O13 nanosheet films are obtained on large-area rigid and flexible substrates in a low-cost, efficient, and environment-friendly manner. The complementary electrochromic device based on LixNa2−xW4O13 nanosheet films exhibits large optical modulation (75% at 700 nm), fast switching responses (2/1.4 s), and outstanding cyclic stability (over 1000 cycles). This aqueous ink can be easily scaled up and exhibits broad application prospects in electrochromism, energy storage, and sensing. This study paves a new methodology to prepare other 2D ion-intercalated materials based on aqueous inks by appropriately selecting ion species.