Facile solvothermal synthesis of plate-like submicron NaNbO₃ particles

Abstract

Plate-like particles of orthorhombic NaNbO₃ were obtained via a solvothermal reaction using a methanol/ethanol mixed solvent, in contrast to the formation of cubic NaNbO₃ particles from methanol alone. Control of the submicron NaNbO₃ particle morphology was achieved by simply changing the solvent, suggesting the role of ethanol in suppressing the growth in one direction.

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The discovery of the Honda–Fujishima effect, i.e., the decomposition of water by titanium dioxide (TiO₂) photocatalysis, prompted researchers to investigate the fundamental properties and practical applications of photocatalysts.^1–4 Titanium oxide (TiO₂) has been widely studied because of its high activity, photo-induced hydrophilicity utilized for the self-cleaning effect, and low cost.^1–4 Additionally, TiO₂ is well-known as a white pigment^2,5–7 that requires submicron-sized particles to achieve its hiding power,^7,8 which is enhanced when the particle shape is platy.⁹ One drawback of TiO₂ as a white pigment is its photocatalytic activity, which causes degradation of fabrics and resins.^1,2 The photo-induced hydrophilicity of sodium niobate (NaNbO₃) was first discovered by Katsumata and co-authors,^10,11 and NaNbO₃ shows potential as a photocatalyst, though with weaker activity than TiO₂,¹⁰ and as a white pigment. The former case is promising for investigating the mechanisms of photo-induced hydrophilicity suppressed by photo-oxidation, while the latter case has high expectations for the particle shape and size of NaNbO₃, which possesses a perovskite-type (ABO₃) structure with an orthorhombic unit cell.¹² Well-defined cubic particles are thus typically obtained.^12–22 Submicron particles are prepared by solvothermal syntheses using synthesized niobium pentoxide (Nb₂O₅) and methanol as the raw materials.^16,17 In contrast, micron-sized and/or aggregated plate-like NaNbO₃ particles are prepared via hydrothermal syntheses^20–24 or topochemical ion exchange reactions of Bi_2.5Na_3.5Nb₅O₁₈, an Aurivillius phase layered perovskite, in molten sodium salts.^25–27 Submicron NaNbO₃ particles with a plate-like morphology are advantageous not only for photocatalytic activity through exposed surface design but also for higher hiding power in white pigment applications.

In this study, we devoted special attention to the facile preparation of plate-like submicron NaNbO₃ particles. The products were prepared by a modified version of the procedure reported in previous studies.^18,19 After dispersing 500 mg of Nb₂O₅ (obtained from Kojundo Chemical Laboratory Co., Ltd.) in 25 mL of methanol (MeOH)/ethanol (EtOH) mixed solvent (1 : 10 vol%), the dispersion was mixed with 5 mL of sodium hydroxide (NaOH) aqueous solution (10 mol L⁻¹). The obtained dispersion was placed in a Teflon vessel with a stainless jacket and then heated at 180 °C for 6, 24, or 48 h. After the reaction, the resultant solids were obtained by centrifugation, exhaustively washed with distilled water and methanol, and then dried at 80 °C for 1 h, to form the products denoted herein as NaNbO₃-(1Me+10Et)-6h, NaNbO₃-(1Me+10Et), and NaNbO₃-(1Me+10Et)-48h, respectively. For comparison, these procedures with a reaction time of 24 h were applied to cases using 25 mL of MeOH, EtOH, or MeOH/EtOH mixed solvent (1 : 4 vol%) instead of the MeOH/EtOH mixed solvent (1 : 10 vol%) to generate products denoted herein as NaNbO₃-Me, NaNbO₃-Et, and NaNbO₃-(1Me+4Et), respectively. In addition, the reactions for MeOH and MeOH/EtOH mixed solvent (1 : 10 vol%) were conducted by changing the amount of Nb₂O₅ from 500 mg to 5 g, to form the products denoted herein as NaNbO₃-Me-5g and NaNbO₃-(1Me+10Et)-5g, respectively. Moreover, the reactions for NaNbO₃-(1Me+10Et) were modified by decreasing the reaction temperature to 150 °C or the NaOH aqueous solution (5 mL) to 2 mol L⁻¹, to generate the products denoted herein as NaNbO₃-(1Me+10Et)-150°C and NaNbO₃-(1Me+10Et)-low-NaOH, respectively. The crystalline phases of the raw material and the products were characterized by X-ray diffraction (XRD) analyses (XRD-6100, Shimadzu). The particle shapes of the raw material and products were characterized by field-emission scanning electron microscopy (FE-SEM) (spra40, Zeiss). The photocatalytic properties of the products were briefly examined by performing decomposition of methylene blue (MB) using 50 mL of MB aqueous solution (1.0 × 10⁻⁵ mol L⁻¹) containing 50 mg of NaNbO₃-Me or NaNbO₃-(1Me+10Et) under 365 nm irradiation for 1 hour. Prior to these experiments, the dispersions were stirred for 2 h in the dark. The photocatalytic experiments were also performed using Degussa P25 standard for comparison purposes. The absorption spectra of NaNbO₃-Me and NaNbO₃-(1Me+10Et) were recorded using a JASCO v-670 spectrophotometer. Before these analyses, the samples were mixed with barium sulfate. The surface areas of NaNbO₃-Me and NaNbO₃-(1Me+10Et) were determined to be 4.8 and 6.0 m² g⁻¹, respectively, from the nitrogen adsorption isotherms measured at −196 °C (Belsorp MINI, MicrotracBEL) using the Brunauer–Emmett–Teller (BET) method.²⁸

Fig. 1 shows XRD patterns for Nb₂O₅ and the products. All the patterns (Fig. 1b–d) show diffraction lines attributed to the orthorhombic phase of NaNbO₃,^13–19 along with the disappearance of reflections due to the orthorhombic phase of Nb₂O₅ (Fig. 1a).^13–19 Notably, the space group for the orthorhombic phase of NaNbO₃ is classified as Pbcm^12,13 or Pbma,^14,15 both are equivalent in different settings. For simplicity, we thus tentatively index the reflections observed in the XRD patterns of the products to those of Pbcm.¹²Fig. 2 presents the FE-SEM images of Nb₂O₅ and the products. The image of NaNbO₃-Me exhibits cube-like particles in the 50–500 nm range along with minute amounts of rod-like particles, while those of NaNbO₃-(1Me+10Et) and NaNbO₃-Et exhibit plate-like particles in the 100–800 nm range (Fig. 2b–d). Compared to the image of NaNbO₃-Et (Fig. 2d), that of NaNbO₃-(1Me+10Et) shows well-defined shapes and plate-like particles (Fig. 2c). Irregular-shaped particles in the 50–400 nm range observed in the image of Nb₂O₅ (Fig. 2a) are absent in all the product images. The absorption spectra of NaNbO₃-Me or NaNbO₃-(1Me+10Et) (Fig. S1†) exhibit broad signals at 320 nm, which is consistent with those reported previously.^16,17 Based on the MB decomposition fractions (Fig. 3), the photocatalytic activities of the present NaNbO₃-Me and NaNbO₃-(1Me+10Et) products were lower than that of P25. In previous studies,^10,11 NaNbO₃ films displayed a lower MB decomposition fraction than TiO₂ films. Also, the 34% MB decomposition fraction for NaNbO₃-(1Me+10Et) was approximately 1.25 times higher than that for NaNbO₃-Me (Fig. 3), in agreement with the BET surface areas for NaNbO₃-(1Me+10Et) (6.0 m² g⁻¹) and NaNbO₃-Me (4.8 m g⁻¹). Therefore, these results show that all the products are truly orthorhombic NaNbO₃ particles.

Fig. 1 XRD patterns for (a) Nb₂O₅, (b) NaNbO₃-Me, (c) NaNbO₃-(1Me+10Et), and (d) NaNbO₃-Et.

Fig. 2 FE-SEM images of (a) Nb₂O₅, (b) NaNbO₃-Me, (c) NaNbO₃-(1Me+10Et), and (d) NaNbO₃-Et.

Fig. 3 MB decomposition fraction for (a) NaNbO₃-Me, (b) NaNbO₃-(1Me+10Et), and (c) P25.

Notably, the plate-like particles form at a relatively early state of the reaction as suggested in the FE-SEM image of NaNbO₃-(1Me+10Et)-6h, which shows smaller and thinner plate-like particles than those of NaNbO₃-(1Me+10Et) (Fig. 2c and S2a†). The plate size reaches a maximum at about 24 h, since there are no major differences in the particle size range or shape observed between the FE-SEM images of NaNbO₃-(1Me+10Et) and NaNbO₃-(1Me+10Et)-48h (Fig. 2c and S2b†). When the MeOH/EtOH ratio in the mixed solvent was changed from 1 : 9 to 1 : 4 vol%, the thickness of the plate-like particles increased from 20–80 nm to 50–250 nm, as shown in the FE-SEM images of NaNbO₃-(1Me+10Et) and NaNbO₃-(1Me+4Et) (Fig. 2c and S2c†). Thus, the thickness of plate-like particles can be controlled by the reaction time or the MeOH/EtOH mixing ratio. In contrast, an increase in the amount of Nb₂O₅ as a raw material causes the formation of cube-like particles as exhibited in the FE-SEM image of NaNbO₃-(1Me+10Et)-5g, which is similar to NaNbO₃-Me-5g (Fig. S2d and e†). Cube-like particles also form when the reaction temperature or the NaOH concentration is decreased, based on the FE-SEM images of NaNbO₃-(1Me+10Et)-150°C and NaNbO₃-(1Me+10Et)-low-NaOH (Fig. S2f and g†). Based on these results, crystal growth in certain directions is suppressed for cubic NaNbO₃ particles under the reaction conditions that form plate-like particles. Feasible mechanisms are discussed below.

It is well-known that hydrothermal reactions of the orthorhombic phase of Nb₂O₅ in the presence of a strong base generate cubic NaNbO₃ with the same phase, with sizes of 50 nm–20 μm,^15–22 while those in the 50–200 nm range are highly aggregated²² and are prepared using relatively small amounts of Nb₂O₅ and dilute NaOH solution.¹⁴ In addition, cubic particles of NaNbO₃ typically form via fibrous sodium niobate hydrate (Na₂Nb₂O₆·nH₂O) as the intermediate.^16,17,20,22 In the literature, possible mechanisms for converting fiber- into cubic-particles have been presented,¹⁶ but there is scant direct evidence for the process involved. Unfortunately, to the best of our knowledge, there have been no reports on the formation of fibrous sodium niobate hydrate (Na₂Nb₂O₆·nH₂O) in solvothermal syntheses using alcohols. In addition, rod-like and orthorhombic NaNbO₃ particles have been synthesized hydrothermally, however, detailed formation mechanism is also absent.²² A prior study found that methanol, with a lower dielectric constant than water, affected the raw material solubility for the solvothermal syntheses of NaNbO₃ as follows,¹⁹ a large number of nuclei form, and the crystal growth is retarded when methanol is used instead of water, causing a decrease in the maximum sizes of cubic NaNbO₃ particles from 600 nm to 2 μm to 100–300 nm.¹⁹ As mentioned above,¹⁷ the particle size of Nb₂O₅ is a possible factor to determine its solubility in a strongly basic solvent. The size of the Nb₂O₅ particles used in the previous studies was 100–500 nm as judged from the FE-SEM images.¹⁹ This size range is similar to that of the present Nb₂O₅ particles, although smaller particles are also present, as shown in the SEM image (Fig. 2a). Therefore, the present results for NaNbO₃-Me in the 50–500 nm range (Fig. 2b) are mostly consistent with previously reported values^18,20 and show that the size distribution of cubic NaNbO₃ particles is wider than the reported values.¹⁷ In the previous reports on the hydrothermal synthesis of plate-like NaNbO₃ particles in the 200 nm to 1 μm range,^20–22 aggregated particles form at relatively short reaction times, whereas longer reaction times generate cubic NaNbO₃ particles. Plate-like particles likely form at an early stage of the hydrothermal synthesis, and the growth rate in the direction to convert plate-like particles into cubic particles seems to be rapid. Judging from the SEM images of the plate-like particles obtained by hydrothermal synthesis,^20–22 they are thinner than those in the present study using solvothermal synthesis (Fig. 2b and c). In the present study, plate-like orthorhombic NaNbO₃ particles form when ethanol is used as the solvent. The dielectric constant of ethanol is lower than that of methanol.²⁷ In addition, the dielectric constant of a mixture of methanol and ethanol decreases with an increasing ethanol content.²⁹ Based on the aforementioned discussion on the solvothermal synthesis of NaNbO₃, the rapid formation and preservation of plate-like particles, and the decrease in their thickness with an increasing ethanol content in the mixed solvent shown in the FE-SEM images (Fig. 2c and S2a–c†),¹⁹ ethanol likely acted to retard the crystal growth more compared to methanol. The unit cell of orthorhombic NaNbO₃ is longer in one direction than in the other directions, i.e., four NbO₆ octahedra bridged via the vertices are longer in one direction.^12,30,31 Based on the crystal structure of NaNbO₃ determined in a previous study,¹² the electron density in the longer direction for orthorhombic NaNbO₃ is lower than that in other directions.¹² Additionally, step- and kink-like structures³⁰ are present on the surfaces of the plate-like particles of NaNbO₃-(1Me+10Et) (Fig. 2c). Therefore, the crystal growth of NaNbO₃ in the ethanol and methanol mixture is likely to be suppressed in certain directions, causing the formation of plate-like particles. The plate-like particles in the present study are thus unlikely to grow into cubic particles, which is consistent with the formation of cube-like particles with increasing amount of raw material that could cause particle to grow in the thickness direction (Fig. 2c and S2d†). In addition, a decrease in the reaction temperature and base concentration may retard the growth of plate-like particles in the lateral direction, thus forming cube-like particles (Fig. 2c and S2f and g†). Such growth suppression in one direction could be utilized in further studies on the design and control of both the thickness and the lateral size of particles with a plate-like morphology.³² In addition, compared to the ethanol and methanol mixture, a larger number of nuclei and a lower solubility of Nb₂O₅, which leads to less availability of the Nb source, may likely cause the formation of smaller and irregular plate-like particles when ethanol is used solely as the solvent. A decrease in the particle size of NaNbO₃ by ethanol addition is feasible judging from previous reports,^18,19,33 although the experimental conditions including the niobium sources in these previous studies are not identical. In addition, the particle size in other ABO₃ perovskites, such as potassium niobate (KNbO₃) and barium titanate, prepared by solvothermal synthesis increases by changing the solvent from methanol to ethanol.^34,35 Thus, reports on the solvothermal syntheses of other ABO₃ perovskites including those using a mixture of methanol and ethanol^36,37 are not very helpful for clarifying the growth mechanisms involved in the present study. Interestingly, although in previous studies the particle morphology of KNbO₃ changed very little when using a mixed solvent,³⁴ in the present study, the NaNbO₃ particle shape was successfully changed from cubic to plate-like. Note that there are non-linear relationships between specific solvent parameters for methanol–ethanol mixtures and the methanol or ethanol content.²⁹ Therefore, the effects of methanol–ethanol mixed solvents on the morphology of NaNbO₃ particles are worth further investigating, and it is thought that fibrous sodium niobate hydrate (Na₂Nb₂O₆·nH₂O) could play a role. Nevertheless, the cubic- and plate-like NaNbO₃ particles identified in the present study, exhibiting photo-inducted properties, may find practical applications.^{10,11,38–40} The photocatalytic activity of plate-like NaNbO₃ could be especially dependent on its morphology because there is a relationship between the MB decomposition fraction and the BET surface area for cubic- and plate-like NaNbO₃, despite the lower water dispersibility of plate-like NaNbO₃ (Fig. S3†). Such studies are ongoing in our laboratory, and the results will be reported in due course.

In summary, we have demonstrated the formation of platy submicron-sized NaNbO₃ particles via solvothermal synthesis using a methanol/ethanol mixed solvent (1 : 10 vol%). The results of the present study could potentially lead to a method for producing a wide range of perovskite compounds⁴¹ with a plate-like morphologies.

Author contributions

Shingo Machida: conceptualization, investigation, data curation, writing – original draft, preparation, and project administration; Shoma Niwa: data curation; Sho Usuki: writing – review and editing; Kazuya Nakata: writing – review and editing; Makoto Ogawa: writing – review and editing; Atsuo Yasumori: project administration; Ken-ichi Katsumata: conceptualization, writing – review and editing, and supervision.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

This research was supported by a Grant-in-Aid from the moon-shot project (JPNP18016) “Development of photo-switching ocean-degradable plastics with edibility” of New Energy and Industrial Technology Development Organization (NEDO), Japan.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ce00665k

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