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. 2024 Mar 22;15(13):3509–3515. doi: 10.1021/acs.jpclett.4c00466

Electron Storage in Monolayer Tungstate Nanosheets Produced via a Scalable Exfoliation Method

Fuminao Kishimoto 1,*, Kazuhiro Takanabe 1
PMCID: PMC11000239  PMID: 38517369

Abstract

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Inorganic nanosheet materials with atomic thinness have been widely studied as (photo)catalytic materials due to their unique electronic states and surface structures. One scalable and reproducible method of producing monolayer nanosheets is a top-down approach based on the exfoliation of layered parent compounds using an alkylammonium solution as a surfactant. However, H2W2O7 layered tungstates dissolve in basic aqueous solutions, making them unsuitable for the exfoliation process. This work proposes a scalable method to obtain monolayer WO3 nanosheets with a very high external field responsiveness. This work shows that H2W2O7 topochemically swells in a concentrated octylamine (C8N17NH2) aqueous solution with a concentration above the solubility of octylamine in water. Water was added for exfoliation of the liquid crystalline phase into isolated W2O72– nanosheets with octylammonium (C8N17NH3+) protection. Crystalline WO3 nanosheets on the n-Si substrate obtained with calcination exhibited electron richness in the conduction band due to static electron transfer at the interface.

Since the creation of single-layer graphene, catalysts and devices have been developed using atomic-layer-thickness nanosheets in recent years.1,2 The unique features of nanosheets are that they have a bulk structure in two dimensions, while almost all atoms are located on the surface or in sublayers. Two-dimensional oxide nanosheets in particular are attracting attention for their ability to serve unique electronic structures that can be applied for functional photocatalysts3 and strong active acid catalysts.4,5 Their applications are expanding to revolutionizing oxide electronics and spintronics, such as energy-storage devices due to their greater availability of chemically active interfaces.6,7 Practically, the oxide nanosheets generally have strong chemical durability and robustness, owing to their covalency and adaptivity to the environment.

The functionalities of nanosheets can be easily controlled by the formation of heterojunction interfaces with other semiconducting or metallic materials.8 In heterojunctions made of ordinary 3D bulk materials, atoms near the interface are far from the surface. This makes it difficult for changes in the electronic structure that result from heterojunctions to directly affect the catalytic reaction mechanism at the surface. However, in two-dimensional nanosheet materials, all atoms are either exposed on the surface or present in sublayers, so the effect of changes in electronic state due to heterojunctions can be expected to be almost uniform throughout the crystal.9 As a result, the formation of heterojunction interfaces can dramatically alter the catalytic activity of two-dimensional nanosheets.10

Scalable synthesis methods for nanosheets can be broadly classified as top-down and bottom-up methods. Top-down methods are based on solution exfoliation of layered parent compounds using amphiphilic molecules as an exfoliation agent (such as quaternary alkylammonium cations),11−13 and bottom-up methods are based on solution synthesis methods using metal salts as starting materials.14,15 Top-down methods using layered compound exfoliation are advantageous in that they yield nanosheets with a well-defined crystal structure and number of atomic layers.

Examples of layered precursor compounds are clay materials,17 layered titanate and niobate,18,19 layered double hydroxides,20 and alkali-metal-incorporated tungstate (Cs6+xW11O36 and Rb4W11O35).21,22 However, to obtain robust nanosheets by exfoliation, it is important to have moderate electrostatic attraction between layers in the layered compound, easy swelling, and chemical stability of the exfoliated nanosheets in a basic solution containing alkylammoniums, which are used as an exfoliation agent. The production of exfoliated pure WO3 nanosheets without any third elements can be expected using Aurivillius-phase layered tungstates (H2W2O7),23,24 but the exfoliation is not easy because tungstates are unstable in basic solutions. Thus, the structure of H2W2O7 is easily transformed into a bulky rod structure or porous structure by dissolution and re-deposition, even when low-basicity alkylamines such as octylamine are used.25,26

There are a few examples of the synthesis of tungstate nanosheets by exfoliation of H2W2O7 using tetrabutylammonium as an exfoliating agent, as well as exfoliation with octylamine after modification of H2W2O7 with a silane coupling agent.27−29 However, the reported nanosheets were highly fragmentated, and their lateral size was less than 100 nm. Tungsten oxide-based nanosheets synthesized by a bottom-up method have been of great interest as photocatalysts in recent years.16,30−34 A scalable synthesis method for stable tungsten oxide nanosheets by exfoliation of layered compounds is expected to provide tungsten oxide nanosheet materials of atomic layer thickness with a well-defined structure, leading to further catalyst development.

This paper reports the stable exfoliation of H2W2O7 in concentrated octylamine-water mixtures. Figure 1(a) shows a diagram of the swelling and exfoliation processes of layered tungstates. In an aqueous solution containing more than 1.9 mol L–1 n-octylamine (the solubility of n-octylamine in water is 1.55 mmol L–1), H2W2O7 swells and enters a liquid crystal phase with oriented W2O72– layers. The concentration of H2W2O7 is decreased by adding water, which allows each layer to be isolated from each other to obtain exfoliated tungstate nanosheets.

Figure 1.

Figure 1

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(a) Illustration of swelling and exfoliation of layered tungstate, H2W2O7, reacted with more than 1.9 mol L–1 of n-octylamine in H2O solvent. (b) Polarizing microscope images of H2W2O7 reacted with 1.9 mol L–1 of n-octylamine in H2O. Inset: Photograph of the dispersion of H2W2O7 with 1.9 mol L–1 of n-octylamine in H2O. (c) SAXS patterns of H2W2O7 dispersed in concentrated n-octylamine/H2O with different H2W2O7 concentrations and (d) d-value of 001 plane diffraction peaks as a function of H2W2O7 concentration. (e) TEM image of exfoliated tungstate nanosheets (W2O72–). The inset shows a selected area electron diffraction pattern.

When the amount of n-octylamine added was 0.078–0.78 mol L–1, suspended particles could be clearly recognized. Figure S2 shows SEM images of parent H2W2O7 and particles collected by centrifugation of the dispersion. H2W2O7 that was reacted with 0.078 mol L–1 n-octylamine (Figure S2(b)) had a particle size of 5–10 μm, which was not significantly different from that of the parent H2W2O7 (Figure S2(a)). When H2W2O7 was reacted with 0.31–0.78 mol L–1 of n-octylamine (Figures S2(c) and (d)), rod-like particles of 1–5 μm in the transverse direction were obtained. The length of the rods exceeded the particle size of the original H2W2O7, suggesting that these rod-like products are produced by a dissolution–re-deposition mechanism, which has been reported previously.25 Thus, the starting material of H2W2O was dissolved into a liquid phase by reaction with diluted n-octylamine.

When the amount of n-octylamine was increased to 1.9 mol L–1, the dispersion became a homogeneous solid that was so viscous that individual particles could not be visually identified. Figure 1(b) shows a polarizing microscope image of the sol. A very long worm-like image with a stripe pattern perpendicular to the long axis was observed. Supporting Video 1 shows the generation of the worm-like structure just after the addition of the n-octylamine/H2O mixture to H2W2O7 powder. The layered structure of H2W2O7 swelled as a result of the addition of the n-octylamine/H2O mixture, and the interlayer distance was increased, forming a liquid crystalline structure. The length of the short-axis direction of the image is consistent with the original particle size of H2W2O7 (5–10 μm, Figure S2(a)).

The development of a liquid crystalline phase due to osmotic swelling of layered compounds in solution has been reported with layered titanates.35 The interlayer space was filled with protonated octylammonium, octylamine, and water, and each layer was aligned to form a liquid crystalline structure. This was likely possible because of balancing the electrostatic repulsion of each anionic W2O72– sheet and the van der Waals force of octylamines/octylammoniums.

Figure 1(c) shows SAXS patterns of the H2W2O7 dispersion with 1.9 mol L–1 of n-octylamine after dilution with various amounts of water. When the H2W2O7 concentration was 40 g L–1, which corresponds to the specimen in Figure 1(b), a diffraction peak at 1.43° was clearly observed. The peak can be assigned to 001 basal reflections of adjacent W2O72– layers in the liquid crystalline phase. The peak was shifted to a lower angle as the concentration of H2W2O7 in the solution decreased, suggesting that the interlayer distance galleries were expanded by increasing the water addition.

Figure 1(d) shows the d-value of the 001 diffraction peak as a function of H2W2O7 concentration. With increasing water addition, the distance between the W2O72– layers drastically increased to 27 nm at 10 g L–1. After further dilution of the H2W2O7 dispersion to 5 g L–1, the diffraction peak disappeared. A polarizing microscope image of the dispersion of H2W2O7 at 5 g of L–1 (Figure S3) shows an isotropic phase. Thus, the W2O72– layers should be completely isolated into nanosheets. Figure 1(e) shows the TEM image of a nanosheet. The crystalline nature of the nanosheets can be confirmed by the selected-area electron diffraction (SAED) pattern, which shows bright spots originating from the diffraction of orthorhombic crystals. Thus, the nanosheets retained their lateral crystal structure in H2W2O7.

Dissolution and re-deposition of layered tungstate proceeded in low concentrations of octylamine solution, while dissolution of tungstic acid was less likely to occur in high concentrations of octylamine solution. As a result, nanosheet exfoliation by topotactic swelling was successfully achieved. Figure S4 shows a photograph of the H2W2O7 dispersion with 1.9 mol L–1 of n-octylamine aqueous solution. This dispersion was like a gel with no fluidity. Polarized microscopy observations suggested that the swollen layered tungstate and large amounts of n-octylamine form a periodic structure. Due to the low fluidity of the matrix, damage of tungstate was less likely to occur. Thus, dissolution and re-precipitation were suppressed compared to in dilute n-octylamine solution.

An aqueous dispersion of W2O72– nanosheets (5 g L–1) was dropped and spin coated onto a single-crystalline heavily doped n-Si substrate (0.001–0.01 Ω cm). AFM topographic observation confirmed the uniform height (2.2 nm) of the sheet-like structure on the Si substrate (Figure 2(a) and (b)). To clarify that the sheet-like structure was composed of a tungsten compound, selected area Auger electron spectra were recorded. Figure 2(c) shows the SEM image of the deposited substrate with the reflected electron mode. The substrate was tilted at 70° from the electron beam to enhance the bumps and dips of the surface structure. Auger electron spectra were recorded within the bright area (#1), showing the signal of W and N derived from tungstate nanosheets and octylammonium (C8H17NH3+) surfactants with Si, C, and O signals, while the dark area (#2) did not exhibit tungsten signals (Figure 2(d)). Therefore, the sheet-like structure observed by AFM was confirmed to be due to tungsten nanosheets.

Figure 2.

Figure 2

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(a) AFM image of exfoliated tungstate nanosheets on n-type Si substrate and (b) height profiles recorded on the white lines described in panel (a). (c) Back scattered electron image of exfoliated tungstate nanosheets on n-type Si substrate and (d) Auger electron spectra recorded in the selected area of panel (c). (e) AFM images of tungstate nanosheets on n-type Si substrate after heat treatment at 350 °C in the air and (f) height profiles on a line described in panel (e). (g) Infrared reflection absorption spectra of tungstate nanosheets on n-type Si substrate before and after heat treatment in the air at 350 °C. (h) Grazing incidence X-ray diffraction pattern of tungstate nanosheets on n-type Si substrate after heat treatment at 350 °C in air.

From the AFM image, the shape of the nanosheet was not a continuous structure but had random pores with a size of 10–500 nm. In addition, the edge of the sheet appeared smooth and amorphous rather than having a structure with clear crystalline planes. It can be considered that such a sheet structure should be formed by partial dissolution of tungstate nanosheets and re-precipitation after the deposition on Si substrates. To confirm the dissolution, the mass spectrum of the solution was measured by ESI–TOF–MS after the tungstate nanosheets were precipitated by salting with NaCl and separated by filtration (Figure S5). The spectrum confirmed the dissolved polytungstic acid in the solution, which should be derived from the partial dissolution of W2O72– nanosheets, resulting in the formation of nanosized pores on the nanosheets.

The measured thickness of the sheet was larger than that estimated from the crystal structure of W2O72– nanosheets, as shown in Figure 1(a), because the AFM height included adsorbed C8H17NH3+ surfactants. The C8H17NH3+ molecules should be adsorbed on both sides of the nanosheet (between the Si substrate and the opposite side). Since the molecular length is approximately 1 nm, the C8H17NH3+ molecules should adsorb on the W2O72– nanosheets with a tilt angle.

The substrate was calcined at 350 °C in air for 2 h to remove the surfactants. Figure 2(e) shows the AFM image of the substrate after the calcination. Although the sheet-like structure with random pores was retained after the calcination, the height of the sheets was decreased to 0.8 nm (Figure 2(f)), which was almost equal to the expected thickness of W2O72– nanosheets on the basis of their crystal structure. Therefore, adsorbed C8H17NH3+ should be mostly removed by air calcination.

The removal of n-octylamine by calcination can also be confirmed by the ATR–IR spectra of the substrates shown in Figure 2(g). The W2O72– nanosheets deposited on the substrate before calcination exhibited a peak at 1141 cm–1, which can be attributed to the NH2 angular vibration peak derived from the C8H17NH3+ surfactants. The peak disappeared after air calcination, indicating that the surfactant C8H17NH3+ molecules were removed by the calcination. Accompanying the removal of the surfactant, the peak at 3270 cm–1 attributed to the O–H stretching vibration mode was enhanced. Therefore, the surface of the W2O72– nanosheets was terminated with protons and formed hydroxyl groups. The peaks at 1058, 957, 868, and 770 cm–1 were observed before and after calcination and can be attributed to W=O and O—W—O vibration modes.

The crystallinity of the calcined tungstate nanosheets on the substrate was evaluated by grazing incidence X-ray diffraction (GI–XRD) measurement (Figure 2(h)). A broad diffraction peak was observed around 2θ = 23°. Bulk WO3 with a monoclinic structure is the most stable crystal structure of WO3 and shows three characteristic diffraction peaks around 2θ = 23°, which are derived from the (200), (020), and (200) planes. The tungstate nanosheets do not have a continuous structure in the c-axis direction, and the broad diffraction peak at 2θ = 23° can be assigned to the (200) or (020) planes. Therefore, these results confirmed the formation of a crystalline WO3 nanosheet on the n-type Si substrate. Based on the crystal structure of W2O72– nanosheets, the WO3 nanosheet has a double octahedral structure where WO6 octahedral structures are stacked in two levels.

Figure 3(a) shows the XPS valence spectra of n-type Si or SiO2-covered Si substrates with or without WO3 nanosheets. The binding energy of the spectra was corrected by the C 1s peak at 285.0 eV (Figure S6). The onset binding energy of the valence spectra corresponds to the energy difference between the Fermi level and the valence band maximum at the surface of the specimen.36−38 The dopant concentration in n-type Si used in the current study can be estimated as the order of ∼1020 cm–3 from the resistivity of 0.001–0.01 Ω cm.39 In such a heavily doped n-Si, the Fermi level should be very close to the conduction band lower level. In fact, the bare n-type Si substrate shows a valence spectra signal starting around 0.9 eV (Figure 3(a)), which was close to the band gap of Si (1.1 eV). The n-type Si substrate’s thermally oxidized SiO2 layer (SiO2/n-Si) showed the threshold of the valence spectrum around 4.2 eV, showing that the valence band maximum in SiO2 was much deeper than that of bare n-type Si.40

Figure 3.

Figure 3

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(a) XPS valence spectra of n-type Si or SiO2-covered n-type Si substrates with or without WO3 nanosheets. (b) Diagram of the electronic structure in the n–n heterojunction of WO3 nanosheets and n-type Si substrate.

The onset binding energy of the valence band spectrum of SiO2/n-Si deposited with WO3 (WO3/SiO2/n-Si) was greatly reduced compared to that of SiO2/n-Si. Because of the deep valence band and insulation property of SiO2, the onset energy can be attributed to the valence band maximum of the WO3 nanosheet having almost no interaction with the substrate. Therefore, the energy gap between the Fermi level and the valence band maximum in WO3 nanosheets can be estimated as ca. 2.2 eV. This value should be reasonable considering the n-type semiconductor properties and the band gap of bulk WO3 (2.7 eV).41

Since the valence band maximum of n-type Si was shallower than that of the WO3 nanosheets, it would be expected that the valence band spectrum of WO3/n-Si would be almost identical to that of n-Si. However, in fact, an onset at 0 eV was observed. Because the spectral shape in the region above 1 eV was a superposition of the spectra of n-Si and WO3 nanosheets, this onset at 0 eV should be assigned to a new electronic state induced by the heterojunction of n-Si and WO3 nanosheets. The expected electronic state is due to static electron transfer from n-Si to WO3 nanosheets induced by Fermi level matching at the n–n heterojunction interface (Figure 3(b)). Thus, free electrons in n-Si were transferred to the conduction band or donor level of the WO3 nanosheet, which caused the onset of the valence spectrum to occur at 0 eV.

Thanks to the atomically thin structure of the WO3 nanosheets, the presence of free electrons induced by the n–n heterojunction could be observed by XPS. Because the thickness of the nanosheet was much smaller than that of a depletion layer (or an accumulation layer) of the n–n heterojunction, the electronic state of the interface could be clearly observed by surface measurement techniques. Figure S8 shows the valence spectrum of bulk WO3 particles (∼200 nm; see SEM image in Figure S7) deposited on an n-type Si substrate. The onset potential of the spectrum was around 1.5 eV, and there were no signals just below the Fermi levels.

The electron accumulation within WO3 nanosheets induced by the n–n heterojunction with n-type Si was evaluated as a detailed spatial distribution using scanning probe microscopy techniques. Kelvin probe force microscopy (KPFM) was used to evaluate the surface potential distribution (Figure 4(a) and (b)). The area with WO3 nanosheet deposition had a clearly negative surface potential compared with the area with the exposed n-Si surface. Comparing the line profiles (Figure 4(c)), the surface potential was about 10 mV lower in the area where the WO3 nanosheets were deposited. Assuming an electrical connection between the WO3 nanosheet and n-Si, this contact potential difference in the KPFM measurement suggests a more negative charge on the WO3 nanosheet surface, indicating the electron accumulation in the WO3 nanosheets induced by the n–n heterojunction with n-Si. A similar surface potential difference in a WO3/n-Si n–n heterojunction structure has been reported.42

Figure 4.

Figure 4

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(a) Topography, (b) surface potential mapping, and (c) line scan results recorded by KPFM measurement and (d) topography, (e) surface capacity mapping, and (f) line scan results recorded by sMIM measurement of WO3/n-Si.

Scanning microwave impedance microscopy (sMIM) was also performed on a similar substrate. sMIM is a non-contact-mode scanning probe microscope method where the local conductivity of a material surface with nanometer scale can be visualized by scanning a cantilever that is connected to a microwave generator (1 GHz in the current study) and a signal detector. In the case of dielectric materials, free charges increase the dielectric constant of the surface, and then the response in the imaginary part of the complex impedance signal increases.43 As shown in Figure 4(d) and (e), deposited areas of WO3 nanosheets showed impedance signals higher than those of exposed n-Si areas. Line scans in Figure 4(f) also show this trend well, with the WO3 nanosheet surface showing impedance signals that are about 40 mV higher than those of n-Si. The results show very high electrical conductivity of WO3 nanosheets bonded to n-Si, which may be due to free electrons flowing into WO3 at the n–n heterojunction interface.

In conclusion, scalable exfoliation of tungstate nanosheets (W2O72–) in a concentrated octylamine aqueous solution was achieved. H2W2O7 changes structure by dissolution and re-precipitation in dilute octylamine aqueous solution, and we found that H2W2O7 topochemically swells without dissolving in aqueous solution containing more than 1.9 mol L–1 of n-octylamine. This results in a liquid crystal phase of each aligned monolayer. The electrostatic interaction between each layer was weakened by dilution of the liquid crystalline phase with water. Thus, it can be exfoliated into isolated W2O72– nanosheets with n-octylamine as a surfactant. Through thermal treatment of monolayer nanosheets on a Si substrate, 0.8 nm-thick WO3 nanosheets were obtained. The thickness was consistent with the structure of monolayer W2O72– nanosheets (a double WO6 octahedral structure). At this heterojunction interface of WO3 nanosheets and the Si substrate, static electron transfer from n-Si to WO3 occurred due to Fermi level matching, resulting in an accumulation of free electrons in the conduction band or donor level of the WO3 nanosheet. This new method for obtaining atomically thin WO3 nanosheets provides good reproducibility, scalability, and electronic activity and is expected to lead to the development of new photocatalysts and sensors.

Acknowledgments

We thank Prof. Y. Wada (Tokyo Institute of Technology) and Prof. S. Tsubaki (Kyushu University) for their private discussions. This work was financially supported by the Grant-in-Aid for Challenging Research (Exploratory) (19K22078).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpclett.4c00466.

  • Experimental section, XRD patterns and SEM images of tungstates, and ESI-TOF-MS signals of H2W2O7 dispersion (PDF)

The authors declare no competing financial interest.

Supplementary Material

jz4c00466_si_001.pdf (923KB, pdf)
jz4c00466_si_002.avi (90MB, avi)

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