The photocatalytic metal-oxides have attracted considerable attention thanks to their outstanding properties and applications. The solar power can be utilized to drive fuel-generating reactions, such as the photocatalytic water-splitting^{[1, 2]}. Among these metal-oxides, tantalate (Ta)-based photocatalysts such as NaTaO₃ has been of much interest because of its high quantum efficiency^[3-5], which can be up to about 56%^[6]. Na₂Ta₄O₁₁ is another type of sodium tantalite. The structure and tantalum coordination environment of Na₂Ta₄O₁₁ are different from NaTaO₃, which provide various insights into its optical and photocatalytic properties. Na₂Ta₄O₁₁ crystal has been shown to be an insulator with an indirect bandgap of 3.63 eV^[7]. Its space group is R3c and is included in a rhombohedral system. The lattice parameters were a = 0.62086 nm and c = 3.6618 nm^[8]. Ratnamala et al. reported that Na₂Ta₄O₁₁ acted as a photocatalyst to promote a water splitting reaction^[9]. To advance the exploration towards its fundamental physical properties, the synthesis of Na₂Ta₄O₁₁ will be essential. Flux synthetic and sol-gel are the main approaches. For example, McLamb et al. prepared large single-crystal Na₂Ta₄O₁₁ particles with a K₂SO₄/Na₂SO₄ flux at a temperature of 1000 °C for 2 h^[10]. In Katsuya Teshima’s study, Na₂Ta₄O₁₁ crystal was grown in Na₂Mo₂O₇ flux via a slow-cooling method ^[11].

However, these methods have always produced impure crystal because they were grown in air, and the direct preparation of product in situ led to mixed reactants and products. Furthermore, these methods obtained either bulk Na₂Ta₄O₁₁ crystals or small crystals. To date, there has been no study on the growth of thin-layered Na₂Ta₄O₁₁, which will be essential to explore its optical properties. Chemical vapor deposition (CVD) is a powerful technology to synthesize large-scale two-dimensional (2D) materials. CVD is a process in which the chemical reactions between gaseous substances occurred, and subsequently produced solid sediments on the substrate^[12-14]. The CVD method can not only control the thickness and size of materials but also prepare crystals with higher purity^[15-17]. Based on this, we believe that the CVD method is suitable for the preparation of large and thin Na₂Ta₄O₁₁, which endows it with high crystallinity for more potential applications.

Here, by using Na₂SO₄ and Ta₂O₅ as precursor, we first applied the CVD method to prepare large and thin-layered Na₂Ta₄O₁₁ flake. The structure of Na₂Ta₄O₁₁ flakes were characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), and Raman techniques, indicating the successful synthesis of Na₂Ta₄O₁₁. The atomic force microscope (AFM) demonstrated the thickness reached ~5.2 nm, which was the thinnest Na₂Ta₄O₁₁ reported so far. High resolution transmission electron microscopy (HRTEM) revealed its high crystallinity and quality. Moreover, we observed that the growth time had strong effects on crystal size and thickness. Our work first synthesized thin-layer Na₂Ta₄O₁₁, which paved the way for further exploration of its potential properties and applications.

The growth was performed with a single temperature heating system, which was a 30 cm one-zone furnace system. Na₂SO₄ powder (14.2 mg, 99.5%, Sinopharm Chemical Reagent Co., Ltd) mixed with Ta₂O₅ powder (39.4 mg, 99.9%, Sigma Aldrich) were put into a crucible, which was placed at the highest temperature zone. A sapphire substrate was placed facedown above the mixed powders. Then the center of the furnace was heated to 850 °C for 43 min (the rate was 20 °C/min), which held for 30 min at 850 °C under 100 sccm high-purity (99.9999%) argon atmosphere. After growth, the tube was cooled naturally.

SEM (Hitachi-S4800) combined with EDS was applied to characterize morphology and elemental composition. AFM (Bruker Dimension Icon) was used to measure the thickness. Raman analysis was conducted under the excitation of a 532 nm laser, which executed with an inVia confocal Renishaw Raman spectrometer. HRTEM (Tecnai G2 F20 S-TWIN) analysis was performed to investigate the structure on an atomic scale. The acceleration voltage was 60 kV to avoid damage samples. Element identification was acquired by using XPS (AXIS-165) equipped with a monochromatic Al K_α source (λ = 1486.6 eV). XRD (Bruker Dimension Icon D8 Advance system) measurements were performed by using Cu K_α radiation (40 kV, 40 mA).

Fig. 1(a) illustrates the CVD setup. In brief, a crucible filled with mixture of Na₂SO₄ and Ta₂O₅ was placed in the center of heating zone. The corresponding molar ratio of precursors was 1 : 1. A sapphire substrate was loaded face down on top of the powder. Then, the heating zone was heated to 850 °C for 43 min (the rate was ~20 °C/min), which held for 30 min at 850 °C under 100 sccm high-purity (99.9999%) argon atmosphere. After growth, the furnace was cooled to room temperature. The growth details are depicted in the experimental section. The Na₂Ta₄O₁₁ presented bilayer structure: a distort TaO₇ pentagonal bipyramid layer, which alternated with another layer of isolated TaO₆ octahedra^[18]. Fig. 1(b) displayed the top view of a single layer of TaO₇ pentagonal bipyramids. Fig. 1(c) shows the polyhedral view of a single TaO₆ octahedron, which bond with six edge-sharing pentagonal bipyramids, and the Na⁺ cations partially occupied an irregular seven-coordinate site. This structure functions as a repeated unit cell to bond with another same structure for regular arrange, forming the Na₂Ta₄O₁₁ crystal^[19].

Considering prior works, we deduced the reaction process between Na₂SO₄ and Ta₂O₅, involving two steps^[20]. First, Na₂SO₄ was thermally decomposed to Na₂O with the gaseous product of SO₂/SO₃, plus some other volatile compounds. Second, under appropriate thermodynamic conditions, the formed Na₂O in step one dissolved Ta₂O₅ to generate Na₂Ta₄O₁₁. The reactions are presented as following chemical equations:

$ {\rm{N}}{{\rm{a}}_2}{\rm{S}}{{\rm{O}}_4}\left( {\rm{s}} \right) = {\rm{N}}{{\rm{a}}_2}{\rm{O}}\left( {\rm{s}} \right) + {\rm{S}}{{\rm{O}}_2}\left( {\rm{g}} \right) + \frac{1}{2}{{\rm{O}}_2}\left( {\rm{g}} \right), $  (1)

$ {\rm{N}}{{\rm{a}}_2}{\rm{O}}\left( {\rm{s}} \right) + 2{\rm{T}}{{\rm{a}}_2}{{\rm{O}}_5}\left( {\rm{s}} \right) = {\rm{N}}{{\rm{a}}_2}{\rm{T}}{{\rm{a}}_4}{{\rm{O}}_{11}}\left( {\rm{s}} \right). $  (2)

Typically, the as-grown Na₂Ta₄O₁₁ flakes are triangles and hexagons. Fig. 2(a) displays the representative optical images of triangle Na₂Ta₄O₁₁, in which the length of thin-layer flake was measured to be ~5.5 μm. It was obvious that each triangle flake showed a different color, revealing the varied thickness, in which the brighter triangles indicates thicker flakes and the darker ones represents thinner flakes (marked with red and yellow, respectively). Then, AFM was conducted to analyze the coarseness and height of representative thin flake (Fig. 2(b)). The height diagram revealed a thickness of ~5.2 nm, which was one of the smallest reported values. The corresponding AFM image indicated a flat and uniform surface of thin-layer Na₂Ta₄O₁₁. Structure confirmation of Na₂Ta₄O₁₁ was determined by XRD characterization (Fig. 2(c)), in which the result showed that positions of all diffraction peaks were consistent with the standard peaks of Na₂Ta₄O₁₁. Strong and sharp peaks were observed, almost without the peaks of impurities, indicating the high crystallinity of Na₂Ta₄O₁₁. It is noted that there is a strong peak at ~ 41.7°, which is assigned to the peak of the substrate of sapphire. Fig. 2(d) shows the Raman spectroscopy of Na₂Ta₄O₁₁ excited with a 532 nm laser at room temperature. Six strong peaks are located at 224.5, 240.8, 294.8, 611, 661, and 1328.3 cm⁻¹, respectively. The peak at 410 cm⁻¹ belongs to the A_1g mode of sapphire substrate. Although it was difficult to compare with other work due to the lack of previous studies about the Raman peak of Na₂Ta₄O₁₁, we can still almost determine that these six strong peaks belong to the Na₂Ta₄O₁₁ flake based on the confirmation of XRD.

Figs. 3(a)–3(c) show the X-ray photoelectron spectroscopy (XPS) spectra of Na 1s, O 1s, and Ta 4f, ascertaining the state of each element in Na₂Ta₄O₁₁. A strong single peak at 1071.9 eV is consistent with Na⁺ (Fig. 3(a))^[21]. The peak at binding energies (BEs) of 531.1 eV was assigned as O 1s (Fig. 3(b))^[22]. The Ta-related peaks of Ta 4f_7/2 and Ta 4f_5/2 were located at BEs of 22.7, 23.9, 26.0, and 28.0 eV (Fig. 3(c)), respectively, which was in the range of reported Ta state^[23]. It can be determined that there were two types of Ta state, based on the analysis and reported work, in which the blue peaks indicated Ta⁵⁺ state, and the yellow peaks represented the Ta²⁺ state. The Ta²⁺ was possibly due to the reduction of Ta⁵⁺ by sulfur gas, which released from the thermal decomposition of Na₂SO₄^[20]. However, the exposure of crystal to the atmosphere prior to characterization created large amount of oxygen, plus the oxygen composition of sapphire substrate (Al₂O₃), resulting in the stoichiometric ratio of Na, Ta and O beyond 2 : 4 : 11. Hence, information about the atomic ratio cannot be further extracted from XPS.

We then performed HRTEM technology to characterize the crystalline structure of flake, and the flake was transferred on TEM grid using the poly(methyl methacrylate) (PMMA)-assisted method^[24]. With an acceleration voltage of 60 kV, a HRTEM image with regular stripes was given (Fig. 3(d)). All of the stripes were aligned in the same direction and with the same lattice spacing, indicating the single crystal nature of Na₂Ta₄O₁₁ flake. The interplanar spacing was measured to be ~0.52 nm, consistent with the lattice constant of (012) plane. Fig. 3(e) presents the corresponding post-processed fast Fourier transformation (FFT) image-processed by the Gatan Digital Micrograph software. It is clear that adjacent bright spots form a hexagon, and the measured specific interplanar distances of 5.2 Å assigned to the (012) planes, which further confirms the high crystallinity of Na₂Ta₄O₁₁. In Fig. 3(g), the EDS image shows the peaks of Ta, Na and O with a little impurity of S, which confirms the chemical composition of Na₂Ta₄O₁₁. A small peak of S was possibly caused by the deposition of a small amount of Na₂SO₄ on the flake surface. The performance of the Na₂Ta₄O₁₁ flake would not be affected because the impurities can be dissolved in water or aqua regia without destroying the structure of the flake. The zone selected by the purple box in Fig. 3(f) displays the hexagonal Na₂Ta₄O₁₁ flake. We used this region to collect element information in the EDS characterization.

These results demonstrate that the growth of thin-layer Na₂Ta₄O₁₁ with CVD method worked extremely well. Of the governing parameters in this work, growth time was an important parameter for Na₂Ta₄O₁₁ growth. At a growth temperature of 850 °C with molar ratio (Na₂SO₄ : Ta₂O₅) of 1 : 1, the growth time varied from 30 to 90 min. The optical images of flakes grown on sapphire corresponding to the deposition time of 30, 50, 60, and 90 min are presented in Figs. 4(a)–4(d). It is clear that the main morphology of Na₂Ta₄O₁₁ flake is hexagonal and triangular. No substantial change of crystal size was shown before the growth time of 60 min. For longer growth time of 90 min, the interconnected Na₂Ta₄O₁₁ flakes grow into a continuous film, which was measured to be 112 μm. At the boundary of the continuous film, we can observe several isolated triangles. This indicates that the formation of the film was initiated from the complete coalescence of flakes. Then AFM characterization was applied to measure the representative thickness of flakes grown at 30, 50, and 60 min, the results are shown in Figs. 4(e)–4(g). The AFM characterization of flakes at growth time of 90 min is not shown because of its height difference: it was thicker than the flakes obtained at a growth time of 60 min. These results can be evidenced by the color evolution of the Na₂Ta₄O₁₁ flake. It is clear that the longer deposition time led to a thicker flake. Thickness increased with time without substantial size change before 60 min, which may result from the stronger chemical reactivity and lower energy barrier of Na₂Ta₄O₁₁ surface when compared to the sapphire surface. Consequently, it resulted in a stack growth on thin Na₂Ta₄O₁₁ flake surface instead of epitaxial lateral growth. The average size and thickness of Na₂Ta₄O₁₁ as a function of growth time is illustrated in Fig. 4(h). In this case, the growth time of 60 min was a turning point to control the growth model of vertical stack growth and coalescence of flakes. We speculate that new seeding molecules were likely to deposit on the top of the bottom layer because of its low thermodynamic barrier on the surface. This barrier increased with thickness, thus Na₂Ta₄O₁₁ molecules would tend to nucleate on the substrate surface and form new stacked grown Na₂Ta₄O₁₁ flakes. Subsequently, adjacent Na₂Ta₄O₁₁ flakes integrated with each other to form a large film.

This study is the first work to grow large 2D Na₂Ta₄O₁₁, which is important for researching its potential application and properties. The physical properties of Na₂Ta₄O₁₁ change dramatically from three dimensional (3D) to 2D because electrons were confined to nanoscale motion in two dimensions^[25], hence the synthesis of 2D Na₂Ta₄O₁₁ would expand its performance. Furthermore, 2D Na₂Ta₄O₁₁ is an insulator material with a very large specific surface area, which may be used as a high-performance load or filter material for promising applications in micro-nano photoelectric catalysis.

In conclusion, thin Na₂Ta₄O₁₁ flakes were first successfully prepared by the CVD method on sapphire substrate. The structure and growth of Na₂Ta₄O₁₁ flakes were confirmed with comprehensive characterization approaches. Controllable thickness and size by regulating growth time gave evidence that the CVD method has advantages in controlling growth of Na₂Ta₄O₁₁. This work is important for advancing the various technological applications of Na₂Ta₄O₁₁, and it offers strategies to synthesize other non-layer metal oxide materials.

The work gratefully acknowledged financial support from the National Natural Science Foundation of China (Nos. 21975067, 21705036), Natural Science Foundation of Hunan Province, China (No. 2018JJ3035).