Hierarchical γ-BaB₂O₄ hollow microspheres: surfactant-assisted hydrothermal formation, phase conversion, optical properties and application as adsorbents

Abstract

Three-dimensional (3D) hierarchical metal borate nanoarchitectures have attracted extensive interest for their wide use owing to their versatile compositions and unique properties, and their rational design and facile synthesis with high uniformity and complex interiors have long remained a great challenge. In this contribution, hierarchical γ-BaB₂O₄ hollow microspheres assembled from one-dimensional (1D) nanorods were successfully synthesized by a facile ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) assisted hydrothermal method (180 °C, 12 h), using Ba(OH)₂·8H₂O and H₃BO₃ as the raw materials. The effects of process parameters were investigated in detail, based on which a feasible formation mechanism was proposed. The hierarchical γ-BaB₂O₄ hollow microspheres exhibited a specific surface area of 46.25 m² g⁻¹. The subsequent mild phase conversion (700 °C, 3.0 h) led to β-BaB₂O₄ hollow microspheres with a well-preserved spherical morphology and high crystallinity, indicating hierarchical γ-BaB₂O₄ and β-BaB₂O₄ hollow microspheres of excellent thermostability. Both the hierarchical γ-BaB₂O₄ and β-BaB₂O₄ hollow microspheres exhibited a transparent characteristic from the ultraviolet to the visible region. Moreover, when employed as adsorbents, the hierarchical γ-BaB₂O₄ hollow microspheres presented satisfactory adsorption properties for the removal of Pb²⁺ from mimic waste water, suggesting the great potential applications of the 3D hierarchical porous or hollow nanoarchitectures of metal borates in the future.

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1. Introduction

Over the past few decades, numerous three-dimensional (3D) hierarchical nanoarchitectures such as hollow,¹ core–shelled,² urchin-like,³ hollow urchin-like,⁴ and flower-like⁵ microspheres with unique structures and remarkable properties have attracted extensive attention owing to their great potential in versatile fields. Although much effort has been made, it is still a challenge to prepare 3D hierarchical nanostructures with desirable nano-building blocks and morphologies as well as expected properties. Among the varieties of 3D hierarchical nanoarchitectures, metal borates have been paid extensive attention due to their versatile compositions and unique properties, and they have been widely employed as adsorbents,⁶ anti-wear additives,⁷ photoluminescence host materials,⁸ micro-/nanoscale nonlinear photonic devices,^9–11 catalyst supports,¹² etc. So far however, in addition to some more frequently encountered one-dimensional (1D) borate nanostructures, there have been just a few 3D hierarchical alkaline earth metal borates reported, including hierarchical porous MgBO₂(OH) superstructures,⁶ hierarchical Ba₂(B₅O₉)Cl·(H₂O)_0.5 microspheres,⁸ flower-like Mg₃B₂O₆,¹³ hierarchical porous Ca(BO₂)₂ microspheres,¹² hierarchical laminar rhombic priceite (Ca₄B₁₀O₁₉·7H₂O) superstructures,¹⁴ and flower-like and urchin-like Mg₃B₂O₆:Eu³⁺.¹⁵ Thus, much more work should be dedicated to the challenging controllable fabrication of 3D metal borate nanostructures with high uniformity and complex interiors through facile synthetic routes.

As is known, inorganic nonlinear optical crystals have been serving as critical materials for fast response and optical fiber communications in modern telecommunications, among which barium borate (β-BaB₂O₄) has been recognized as one of the most important nonlinear optical (NLO) materials owing to its characteristic properties. For example, β-BaB₂O₄ exhibited wide optical transmission from 189 nm to 3500 nm, a large effective second-harmonic-generation (SHG) coefficient (d₂₂ = ±2.3 pm V⁻¹, d₃₁ = ±0.16 pm V⁻¹), and a high damage threshold of 10 GW cm⁻² for 0.1 ns pulse-width at 1064 nm, as well as excellent mechanical performances.¹⁶ Also, β-BaB₂O₄ nanoparticles (NPs) could be employed as potential candidates for optical limiting applications in continuous wave (cw) and pulsed laser regimes due to their high linear transmittance and high laser damage threshold. To date, however, only a few attempts to prepare β-BaB₂O₄ nanostructures have been reported. Qu et al. reported an organic-free hydrothermal synthesis for optical quality single-crystal β-BaB₂O₄ microwires and nanowires.¹⁰ Liu et al. carried out controllable synthesis of photoluminescent europium-doped β-BaB₂O₄ nanorods, nanowires, and flower-like assemblies.¹⁷ Li et al. synthesized β-BaB₂O₄ nanospindles through heat treatment of Ba₃B₆O₉(OH)₆ nanorods at 810 °C for 3.0 h.¹⁸ Zhang et al. fabricated single-crystalline β-BaB₂O₄ nanorods by a hexadecyl trimethyl ammonium bromide (CTAB) assisted hydrothermal method, followed by a subsequent annealing.¹⁹ Taking the important applications and great potential of 3D nanoarchitectures into consideration, it is highly desirable to develop a rational, facile, and controllable route to 3D hollow BaB₂O₄ nanoarchitectures.

Herein, to the best of our knowledge, we reported for the first time the successful synthesis of uniform hierarchical γ-BaB₂O₄ hollow microspheres via a facile surfactant-assisted hydrothermal route, and also high crystallinity β-BaB₂O₄ hollow microspheres with well-preserved spherical morphology via subsequent mild phase conversion. The effects of process parameters, such as reactant concentration, surfactant, temperature and time, on the hydrothermal products as well as the EDTA-2Na assisted formation mechanism were investigated in detail. In addition, the phase conversion, optical properties and potential application of the as-synthesized hierarchical γ-BaB₂O₄ hollow microspheres as adsorbents for heavy metal Pb²⁺ ion removal were also presented. The results indicated that the surfactant and Ostwald ripening mechanism played the key role in the hydrothermal self-assembly for the hierarchical γ-BaB₂O₄ hollow microspheres, which exhibited a specific surface area of 46.25 m² g⁻¹. The mild phase conversion led to high crystallinity β-BaB₂O₄ hollow microspheres, and both the hierarchical γ-BaB₂O₄ and β-BaB₂O₄ hollow microspheres revealed unique optical properties. In addition, to explore the novel properties and potential applications of the hierarchical hollow barium borate nanoarchitectures in versatile fields, we have focused on and reported for the first time (as far as we know) the feasibility of using the as-obtained uniform γ-BaB₂O₄ hollow microspheres as an adsorbent. As confirmed, the hierarchical γ-BaB₂O₄ hollow microspheres exhibited a relatively satisfactory performance as the adsorbent for the removal of Pb²⁺ ions from mimic waste water.

2. Experimental

2.1 Synthesis of the hierarchical γ-BaB₂O₄ hollow microspheres

All reagents were of analytical grade and used directly without further purification. In a typical procedure, 3.150 g of Ba(OH)₂·8H₂O was dissolved in 20.0 mL of deionized (DI) water, and 1.237 g of H₃BO₃ (molar ratio: Ba : B = 1 : 2) was dissolved in 30.0 mL of DI water. Then, 0.930 g of EDTA-2Na powder and the H₃BO₃ solution were sequentially added to the Ba(OH)₂ solution under vigorous magnetic stirring at room temperature. Having been stirred for 15 min, the resultant slurry was transferred into a Teflon-lined stainless steel autoclave with a capacity of 63.0 mL. Then the autoclave was sealed, heated to 180 °C (heating rate: 5 °C min⁻¹) and kept in an isothermal state for 12.0 h. After the hydrothermal synthesis, the autoclave was cooled down to room temperature naturally. The as-obtained precipitate was washed with DI water and ethanol three times, then filtered, and finally dried at 70 °C for 12.0 h. To evaluate the effects of the process parameters on the hydrothermal products, the reactant concentration, amount of EDTA-2Na, temperature, and time were tuned within the range of 0.05–0.40 mol L⁻¹, 0–0.10 mol L⁻¹, 120–210 °C and 1.0–18.0 h, respectively, with other conditions unchanged.

2.2 Phase conversion of γ-BaB₂O₄ hollow microspheres

The as-synthesized γ-BaB₂O₄ powder was transferred to a porcelain boat located in a horizontal quartz tube furnace, which was then heated to 500–700 °C (heating rate: 2.5 °C min⁻¹) and kept in an isothermal state for 3.0 h. After calcination, the product was cooled down to room temperature naturally within the tube furnace. Then, the as-obtained product was washed with DI water and ethanol three times, then filtered, and ultimately dried at 70 °C for 12.0 h.

2.3 Hierarchical γ-BaB₂O₄ hollow microspheres as an adsorbent for Pb²⁺ removal

The as-obtained hierarchical γ-BaB₂O₄ hollow microspheres were evaluated for the removal of Pb²⁺ ions from mimic waste water. In a typical procedure, 20 mg of the γ-BaB₂O₄ hollow microspheres was mixed with 50 mL of Pb²⁺-containing solution (concentration: 50.0 mg L⁻¹) under magnetic stirring at room temperature, with the stirring time ranging from 5.0 to 300.0 min. After the adsorption systems had been stirred for their designated times, the resultant slurries were filtered individually, leading to a series of transparent filtrates. The amount of Pb²⁺ absorbed onto the adsorbent was calculated using eqn (1):where q_t (mg g⁻¹) is the adsorption capacity at time t, c₀ (mg L⁻¹) is the initial concentration of the Pb²⁺ solution, c_t (mg L⁻¹) indicates the concentration of the Pb²⁺ solution at any time t, v (L) is the volume of the Pb²⁺ solution, and m (g) is the mass of adsorbent employed.

2.4 Characterization

The crystal structures of the samples were identified using an X-ray powder diffractometer (XRD, MiniFlex600, Rigaku, Japan) with Cu-K_α radiation (λ = 1.5406 Å) and a fixed power source (30 kV, 10.0 mA). The morphology and microstructure of the samples were examined using field emission scanning electron microscopy (SEM, JSM 6700F, JEOL, Japan, at 5.0 kV), and high resolution transmission electron microscopy (TEM, JEM-2010, JEOL, Japan, at 100.0 kV). The size distribution of the microspheres was estimated by directly measuring ca. 100 particles from the typical SEM images. N₂ adsorption–desorption isotherms were measured at 77 K using a chemisorption–physisorption analyzer (Autosorb-IQ2-MP, Quantachrome, USA) after the samples had been degassed at 300 °C for 1.0 h. The specific surface area was calculated from the adsorption branches in the relative pressure range of 0.05–0.30 using the multipoint Brunauer–Emmett–Teller (BET) method, and the pore size distribution was evaluated from the N₂ desorption isotherm using the Barrett–Joyner–Halenda (BJH) model. The chemical bonds in the molecules of the hydrothermal product were determined by Fourier transform infrared spectroscopy (FT-IR, Nexus 470, Nicolet, USA). The optical properties were determined using a UV-vis spectrophotometer (UV-756 CRT, Shanghai Yoke Instrument and Meter Co., LTD, China). The actual residual amount of Pb²⁺ ions within the filtrate was determined by inductively coupled plasma analysis (ICP, Optima 4300DV Spectrometer, PerkinElmer instruments, USA).

3. Results and discussion

3.1 Structure and morphology characterization of the hierarchical γ-BaB₂O₄ hollow microspheres

The structure and morphology related information on the hydrothermal product obtained by heating at 180 °C for 12.0 h, with a molar ratio of Ba : B of 1 : 2 and reactant concentration of Ba(OH)₂ of 0.20 mol L⁻¹, is shown in Fig. 1. Most of the diffraction peaks of the XRD pattern coincide well with those of γ-BaB₂O₄ (JCPDS no. 35-0181) (Fig. 1(a)), with sporadic peaks of low intensity located at 2θ = 13.90° and 41.32° which failed to be indexed. Those few detected diffraction peaks for the present samples were also found to be distinctly present for γ-BaB₂O₄ micro-/nanowires.²² The SEM characterization showed that the as-obtained product particles exhibited a uniform microsphere morphology (Fig. 1(b)). Interestingly, the high magnification SEM image indicated typical hydrothermally synthesized γ-BaB₂O₄ microspheres of hollow structure. The hollow microspheres consisted of 1D nanorods, with a shell thickness of ca. 2.15 µm (Fig. 1(b₁)). Meanwhile, the statistical data revealed that ca. 73% of the as-synthesized uniform microspheres had a size ranging from 12.0–18.0 µm, and the average size of the microspheres was 14.7 µm (Fig. 1(b₂)). The TEM image showed that the constituent nanorods derived from the microspheres, owing to the ultrasonic dispersion during the TEM sample preparation, exhibited a distinct, straight rod-like shape with a smooth surface (Fig. 1(c)), a length of 56–651 nm and a diameter of 14–40 nm. The HRTEM image (Fig. 1(e)) recorded from the red dashed rectangular region in Fig. 1(d) demonstrated explicit lattice fringes with an interplanar spacing of 0.485 nm (Fig. 1(e)), which was somewhat similar to the standard value of the crystal planes (0.452 nm) corresponding to the diffraction peak located at 2θ = 19.62°. This confirmed the presence of as-obtained γ-BaB₂O₄ hollow microspheres of relatively high crystallinity.

Fig. 1 XRD pattern (a), SEM images (b and b₁), TEM (c) and HRTEM (d and e) images, as well as size distribution (b₂) of the γ-BaB₂O₄ hollow microspheres hydrothermally synthesized at 180 °C for 12.0 h with a molar ratio of Ba : B of 1 : 2, and concentrations of Ba(OH)₂ and EDTA-2Na of 0.20 mol L⁻¹ and 0.05 mol L⁻¹, respectively. Vertical lines in (a): the standard pattern of γ-BaB₂O₄ (JCPDS no. 35-0181).

3.2 Effects of temperature and time on the hydrothermal products

To understand the hydrothermal growth of the hierarchical γ-BaB₂O₄ hollow microspheres, temperature- and time-dependent experiments were carried out. As shown in Fig. S1,† the reaction temperature had great influence on the compositions and the morphologies of the products. The crystallinity of the hydrothermal products got higher with the increase in the temperature from 120 °C to 210 °C (Fig. S1(a₁)–(a₃)† and 1(a₁)). When hydrothermally treated at 120 °C, only low crystallinity or almost amorphous (Fig. S1(a₁)†) irregular NPs were obtained (Fig. S1(b)†). When treated at 150 °C to 210 °C, the products were a pure phase of γ-BaB₂O₄ (JCPDS no. 35-0181) (Fig. S1(a₂) and (a₃)† and 1(a₁)). Apparently, with the temperature going up to 150 °C, γ-BaB₂O₄ hollow microspheres emerged (Fig. S1(c)†). The hollow microspheres consisted of short 1D nanostructured subunits (Fig. S1(c₁)†), but with a wide diameter distribution within the range of 12.0–30.0 µm (average diameter: 21.0 µm). As mentioned above, when treated at 180 °C, the as-obtained γ-BaB₂O₄ hollow microspheres exhibited relatively uniform morphology and ca. 73% of them had a diameter within the range of 12–18 µm (Fig. 1(b) and (b₂)). With the temperature going up further to 210 °C, the products were composed of γ-BaB₂O₄ hollow microspheres (diameter: 10.0–20.0 µm) and assembled microrods (Fig. S1(d) and (d₁)†).

With the hydrothermal treatment time increased from 1.0 h to 3.0 h, 12.0 h, and 18.0 h, the majority of the hydrothermal products synthesised at 180 °C were all confirmed as γ-BaB₂O₄, with the crystallinity getting higher and higher (Fig. S2(a)†). Meanwhile, the product particles almost all emerged as microspheres and the average diameter of the microspheres tended to be larger with the elongation of the time (Fig. S2(b)–(d)†). Also, as shown, even when treated at 180 °C for 1.0 h, γ-BaB₂O₄ microspheres were formed (Fig. S2(b)†). When treated for a longer time, more distinct microspheres with hollow structures appeared (Fig. S2(c)† and 1(b)). When treated for an excessive longer time such as 18.0 h, however, the occurrence of some discrete NPs was observed (Fig. S2(d)†). With the aim of uniform hollow γ-BaB₂O₄ microspheres, the optimum hydrothermal time was fixed at 12.0 h.

3.3 Effect of reactant concentration on the hydrothermal products

To investigate the effect of reactant concentration on the composition and morphology of the hydrothermal products, the concentration of Ba(OH)₂ was tuned within the range of 0.05–0.40 mol L⁻¹, with other conditions kept the same. As shown in Fig. 2(a₁)–(a₃), all the products coincided well with γ-BaB₂O₄. Apparently, with the increase in the concentration from 0.05–0.40 mol L⁻¹, the crystallinity of the product became lower first and then somehow higher. Significantly, when the reactant concentration was 0.05 mol L⁻¹, the product was confirmed as γ-BaB₂O₄ urchin-like microspheres, which consisted of radially-grown 1D nanorods (Fig. 2(b)) with a length of ca. 20 µm and a smooth surface (Fig. 2(b₁)). With the concentration increased to 0.10 mol L⁻¹, the product turned to be γ-BaB₂O₄ microspheres whereas with non-uniform diameters (Fig. 2(c)), which were constituted of short nanorods-like subunits (Fig. 2(c₁)). With the concentration increased to 0.20 mol L⁻¹, uniform hollow microspheres were obtained, as previously shown in Fig. 1(b). Nevertheless, when the concentration was further doubled to 0.40 mol L⁻¹, the product became almost discrete irregular nanorods (Fig. 2(d) and (d₁)). As reported, a relatively high degree of supersaturation originating from the high reactant concentration could favor the 3D growth or self-assembly of the microspheres.²⁰ In the present case, the optimum concentration of Ba(OH)₂ for the formation of γ-BaB₂O₄ hollow microspheres was confirmed as 0.20 mol L⁻¹, as demonstrated above in Fig. 1.

Fig. 2 Influence of concentration on the composition (a) and morphology (b–d, b₁, c₁ and d₁) of the products hydrothermally synthesized at 180 °C for 12.0 h, with a molar ratio of Ba : B of 1 : 2 and concentration of EDTA-2Na of 0.05 mol L⁻¹. Concentration of Ba(OH)₂ (mol L⁻¹): a₁, b, b₁ – 0.05; a₂, c, c₁ – 0.10; a₃, d, d₁ – 0.40. Vertical lines in (a): the standard pattern of γ-BaB₂O₄ (JCPDS no. 35-0181).

3.4 Effect of EDTA-2Na on the hydrothermal products

To explore the effect of EDTA-2Na on the composition and morphology of the hydrothermal products, the concentration of EDTA-2Na was tuned within the range of 0.00–0.10 mol L⁻¹, with other conditions remaining unchanged. As shown in Fig. 3(a₁), the hydrothermal product synthesized in the absence of EDTA-2Na was the pure phase of monoclinic Ba₃B₆O₉(OH)₆ (JCPDS no. 01-071-2501)¹⁸ with nano-/microrods or plate-like morphology (Fig. 3(b)). With the concentration of EDTA-2Na changed from 0.025 mol L⁻¹ to 0.038 mol L⁻¹ and 0.050 mol L⁻¹, all the hydrothermal products consisted of the pure phase of γ-BaB₂O₄ and the crystallinity of the products did not change much (Fig. 3(a₂) and (a₃) and 1(a)).

Fig. 3 Effect of the surfactant EDTA-2Na on the composition (a) and morphology (b–e and d₁) of the products hydrothermally synthesized at 180 °C for 12.0 h, with a molar ratio of Ba : B of 1 : 2 and concentration of Ba(OH)₂ of 0.20 mol L⁻¹. Concentration of EDTA-2Na (mol L⁻¹): a₁, b – 0.0; a₂, c – 0.025; a₃, d, d₁ – 0.038; a₄, e – 0.100. Vertical lines in (a): the standard pattern of γ-BaB₂O₄ (JCPDS no. 35-0181).

However, a distinct change in the product morphology emerged with a slight variation of the concentration of EDTA-2Na. When the concentration was 0.025 mol L⁻¹, the product was detected as microrods and irregular NPs (Fig. 3(c)). With the concentration increased to 0.038 mol L⁻¹, distinct self-assembled γ-BaB₂O₄ hollow microspheres and nonuniform assemblies appeared (Fig. 3(d) and (d₁)). When the concentration was increased to 0.050 mol L⁻¹, uniform γ-BaB₂O₄ microspheres were obtained as shown previously (Fig. 1). Nevertheless, with the concentration of EDTA-2Na further increased to 0.10 mol L⁻¹, the composition of the product failed to be indexed at present (Fig. 3(a₄)). The product contained bulky blocks and also distinct, bigger, compact microspheres constructed from NPs (Fig. 3(e)). Thus, the concentration of the surfactant EDTA-2Na played a key role in the hydrothermal formation of the uniform γ-BaB₂O₄ microspheres, and the optimal concentration was as 0.050 mol L⁻¹.

3.5 Shape evolution of the hydrothermal product

Since the transformation of hierarchical γ-BaB₂O₄ hollow microspheres at 180 °C was too fast, a series of time dependent experiments were carried out at 150 °C so as to better understand the hydrothermal growth of the unique γ-BaB₂O₄ hollow microspheres. As shown, the room-temperature coprecipitate was almost amorphous NPs (Fig. 4(a₁) and (b)), and the hydrothermal product grown at 150 °C for 120 min was irregular NPs with very low crystallinity (Fig. 4(a₂) and (c)). Taking the XRD patterns of the room-temperature coprecipitated NPs (Fig. 4(a₁)), reactants (Fig. S3(a)–(c)†) and EDTA–Ba (complex) (Fig. S3(d)†) into consideration, it was obvious that the phase transformation of γ-BaB₂O₄ had gradually taken place, and the hydrothermal treatment led to somehow bigger NPs. With the time extended to 130 min, some relatively loose microspheres with a diameter of ca. 25.0 µm emerged, which were obviously self-assembled from multitudes of tiny NPs (Fig. 4(a₃), (d) and (d₁)). As the time was prolonged to 150 min, relatively high crystallinity and distinct compact γ-BaB₂O₄ microspheres and hemispheres were formed, with an average size of ca. 24.5 µm (Fig. 4(a₄), (e) and (e₃)). Simultaneously, some slender nanorods appeared on the surfaces of the γ-BaB₂O₄ spheres (Fig. 4(e₁) and (e₂)). When the hydrothermal time was further prolonged to 180 min, distinct γ-BaB₂O₄ hollow microspheres were obtained (Fig. 4(a₅) and (f)). Apparently, the γ-BaB₂O₄ hollow structures were formed and the rod-like subunits became thicker and shorter (Fig. 4(f₁) and (f₂)), and the microspheres exhibited an average diameter of ca. 21.6 µm (Fig. 4(f₃)), somehow larger than those uniform γ-BaB₂O₄ hollow microspheres acquired at 150 °C for 720 min (Fig. S1(c) and (c₂)†) with a wide size distribution within the range of 12.0–30.0 µm (average diameter: 21.2 µm). Thus, with the extension of the hydrothermal treatment time, the self-assembled microspheres became more compact, and the average diameter decreased on the whole with the increase in the hydrothermal treatment time from 150 to 180 and 720 min.

Fig. 4 XRD patterns (a), SEM images (b–f, d₁, e₁, e₂, f₁ and f₂) and size distribution (e₃ and f₃) of the products hydrothermally synthesized at 25 °C (a₁ and b) or 150 °C (a₂–a₅, d₁, e₁–e₃, f₁–f₃ and c–f) for different times, with a molar ratio of Ba : B of 1 : 2, and concentrations of the surfactant EDTA-2Na and Ba(OH)₂ of 0.05 mol L⁻¹ and 0.20 mol L⁻¹, respectively. Time (min): a₁, b – 15; a₂, c – 120; a₃, d, d₁ – 130; a₄, e, e₁, e₂, e₃ – 150; a₅, f, f₁, f₂, f₃ – 180.

3.6 EDTA-2Na assisted formation mechanism of the hierarchical γ-BaB₂O₄ hollow microspheres

Based on the above experimental results, a possible formation mechanism of the hierarchical γ-BaB₂O₄ hollow microspheres is proposed. From a chemical point of view, the dissolution of Ba(OH)₂·8H₂O, H₃BO₃ and EDTA-2Na led to the corresponding aqueous ions first, as shown in eqn (2)–(4).

Ba(OH)₂·8H₂O(s) → Ba²⁺(aq.) + 2OH⁻(aq.) + 8H₂O (2)

H₃BO₃(s) + H₂O → B(OH)₄⁻(aq.) + H⁺(aq.) (3)

EDTA-2Na(s) → EDTA²⁻(aq.) + 2Na⁺(aq.) (4)

EDTA²⁻(aq.) + Ba²⁺(aq.) → EDTA–Ba (complex) (5)

Ba²⁺(aq.) + 2B(OH)₄⁻(aq.) + 2H⁺(aq.) + 2OH⁻(aq.) → BaB₂O₄(s) + 6H₂O (6)

EDTA–Ba (complex) + 2B(OH)₄⁻(aq.) + 2H⁺(aq.) → BaB₂O₄(s) + 6H₂O + EDTA²⁻(aq.) (7)

Ba(OH)₂·8H₂O(s) + 2H₃BO₃(s) → BaB₂O₄(s) + 12H₂O (8)

Since Ba(OH)₂·8H₂O has a very low solubility in DI water, even under vigorous magnetic stirring at room temperature, the powder added could not be readily dissolved. With the introduction of EDTA-2Na (Fig. S4(a)†), the EDTA²⁻ ions could immediately chelate the newly dissociated Ba²⁺ ions, resulting in the chelate EDTA–Ba (complex) (Fig. S4(b)†) due to the strong chelation between Ba²⁺ and EDTA²⁻ ions (eqn (5)). In the chelate complex, EDTA²⁻ provided two nitrogen atoms and four oxygen atoms, bringing about tightly wrapped five-membered chelate rings with high stability. Accordingly, the concentration of the free Ba²⁺ ions in the solution was reduced, which promoted the dissolution of Ba(OH)₂·8H₂O. Then, with the dropwise addition of H₃BO₃ solution into the above system, the free Ba²⁺ ions were combined with OH⁻, H⁺ and B(OH)⁴⁻ ions within the solution, giving rise to the more stable γ-BaB₂O₄ phase (eqn (6)). When the slurry derived from the room temperature coprecipitation was transferred into the stainless steel autoclave for hydrothermal treatment at a specific temperature (higher than 150 °C) for a specific time (longer than 130 min), the chelate complex EDTA–Ba gradually decomposed^21,22 and the quantity of the γ-BaB₂O₄ phase gradually increased, as shown in eqn (7). Finally, the overall reaction could be written out, as described in eqn (8).

According to all the above results and analysis, a feasible mechanism was proposed addressing the EDTA-2Na assisted formation of hierarchical γ-BaB₂O₄ hollow microspheres (Fig. 5). Firstly, dissolution of the solid powder Ba(OH)₂·8H₂O in DI water led to aqueous Ba²⁺ and OH⁻ ions, some of which remained in the solid state owing to their corresponding low solubility at room temperature (Fig. 5(a)). Secondly, addition of EDTA-2Na into the solution resulted in the coordinate compounds of the EDTA–Ba complex due to the strong chelation between Ba²⁺ ions and EDTA²⁻ (Fig. 5(b)). Thirdly, addition of B(OH)₄⁻ into the solution facilitated the room temperature coprecipitation of the aqueous ions and EDTA–Ba, bringing about the original nucleation and primary growth of the amorphous NPs (Fig. 5(c)). Subsequently, hydrothermal treatment gradually promoted the phase conversion of the above amorphous NPs (Fig. 5(d)) and also the occurrence of self-assembled loose microspheres with EDTA²⁻ preferentially adsorbed on the surfaces (Fig. 5(e)). With the hydrothermal time prolonged, the constituent NPs began to crystallize into nanorod-like subunits due to the preferential growth, leading to relatively compact hierarchical γ-BaB₂O₄ microspheres (Fig. 5(f)). Finally, when the reaction time was further extended, the nanorod-like subunits further grew into thicker nanorods, attributed to the Ostwald ripening, simultaneously giving rise to the uniform hollow microspheres (Fig. 5(g)). The subsequent cooling, washing and drying enabled the ultimate formation of the hierarchical γ-BaB₂O₄ hollow microspheres.

Fig. 5 EDTA-2Na assisted formation mechanism of γ-BaB₂O₄ hollow microspheres. (a) Dissolution of the solid powder Ba(OH)₂·8H₂O in DI water led to aqueous Ba²⁺ and OH⁻ ions; (b) EDTA–Ba complex obtained with addition of EDTA-2Na into the solution; (c) addition of B(OH)₄⁻ into the solution resulted in room temperature coprecipitation of the aqueous ions and EDTA–Ba, bringing about the nucleation and primary growth of the amorphous NPs; (d) hydrothermal treatment gradually promoted the phase conversion of the amorphous NPs; (e) further hydrothermal treatment led to the occurrence of self-assembled loose microspheres with EDTA preferentially adsorbed on the surfaces; (f) with the hydrothermal time prolonged, the constituent NPs exhibited preferential growth, producing relatively compact hierarchical γ-BaB₂O₄ microspheres consisting of nanorod-like subunits; (g) with the hydrothermal time further extended, the nanorod-like subunits further grew into thicker nanorods, attributed to the Ostwald ripening, and the subsequent cooling, washing and drying enabled formation of the final hierarchical γ-BaB₂O₄ hollow microspheres.

As confirmed above, the surfactant EDTA-2Na played a key role as a chelating and capping agent for the self-assembly of the NPs at the early stage of the hydrothermal treatment, leading to the relatively compact microspheres, very similar to the hydrothermal formation of hierarchical Ba₂(B₅O₉)Cl·(H₂O)_0.5 (ref. 8) and mesoporous SrCO₃ (ref. 23) microspheres. As is known, EDTA²⁻ has four carboxylic groups and two lone pairs of electrons on two nitrogen atoms as binding sites.²⁴ The carboxyl groups afford a firm spatial symmetric configuration, constraining the Ba²⁺ within the symmetric regions. Based on this, by using EDTA-2Na as the surfactant, self-assembled loose microspheres were facilely acquired via the selective adsorption of EDTA²⁻ on the surfaces of the originally formed amorphous NPs. The weak interactions between the amorphous NPs with EDTA²⁻ adsorbed on the surfaces provided the driving force, and the hydrothermal treatment obviously promoted the self-assembly of the loose hierarchical microspheres. Subsequent hydrothermal treatment enabled the preferential growth of the constituent NPs and the evolution from the loose microspheres to the relatively compact γ-BaB₂O₄ microspheres, owing to the traditional Ostwald ripening mechanism. In addition, according to the previous experimental results and analysis, it was believed that Ostwald ripening was also responsible for the transformation from relatively compact microspheres to hollow ones.^25,26 The outer crystalline shells grew on the solid particles accompanied by continuous dissolution and recrystallization of the interior structures.²⁷ During this process, the inner crystallites, which had a higher surface energy, would dissolve and transfer outwards, producing channels connecting the inner space and outer space in the shells.²⁸ This hollowing process was quite similar to that of the SnO₂ hollow nanostructures²⁹ and VO₂ hollow microspheres.³⁰ Thus, on the whole, the synthesis (including room temperature coprecipitation and hydrothermal conversion) and hydrothermal self-assembly, as well as the subsequent Ostwald ripening, contributed to three individual and sequential processes that dominate the formation of the hierarchical hollow microspheres.

3.7 Phase conversion of γ-BaB₂O₄ hollow microspheres to β-BaB₂O₄ hollow microspheres

As is known, the low-temperature phase γ-BaB₂O₄ can be transformed by annealing to β-BaB₂O₄ at 590–650 °C, and then to the high-temperature phase α-BaB₂O₄ at 870–940 °C.^31,32 Accordingly, the present γ-BaB₂O₄ hollow microspheres were calcined at 500 °C, 600 °C and 700 °C for 3.0 h in air so as to see the thermostability of the samples, as shown in Fig. 6. When calcined at 500 °C, no distinct change in the product composition was observed (Fig. 6(a₁)), and the calcined γ-BaB₂O₄ hollow microspheres (Fig. 6(b) and (b₁)) exhibited a relatively broader size distribution, ca. 88% of them having a diameter within the range of 10.0–16.0 µm (average size: 13.7 µm) (Fig. 6(b₂)), in contrast with those hydrothermally synthesized at 180 °C for 12.0 h (Fig. 1(a)). When calcined at 600 °C, in addition to the majority phase of γ-BaB₂O₄ (JCPDS no. 35-0181), rhombohedral β-BaB₂O₄ (JCPDS no. 80-1489) began to appear (Fig. 6(a₂)). Meanwhile, the product morphology did not demonstrate a significant change (Fig. 6(c) and (c₁)), whereas ca. 90% of the calcined microspheres exhibited a diameter of 10.0–16.0 µm (average size: 13.4 µm) (Fig. 6(c₂)). With the temperature increased to 700 °C, the product was confirmed as the pure phase of rhombohedral β-BaB₂O₄ (Fig. 6(a₃)), and 91.0% of the as-obtained microspheres (Fig. 6(d) and (d₁)) revealed a diameter of 9.0–15.0 µm (average size: 11.2 µm) (Fig. 6(d₂)). The TEM images in Fig. 6(e) and (e₁) clearly indicated that the constituent nanorods, which were dissociated from the calcined microspheres during the TEM sample preparation owing to the ultrasonic dispersion, possessed rather rough surfaces, attributed to the phase conversion taking place during the high-temperature annealing. In addition, the high-resolution TEM (HRTEM) image was also recorded from the top end of an individual nanorod (Fig. 6(f)). Corresponding with the red dashed rectangular region, the interplanar spacing of 0.311 nm detected from the legible lattice fringes along the radial direction of the selected nanorod (Fig. 6(f₁)) was quite analagous to the standard value (0.313 nm) of the (220) planes for rhombohedral β-BaB₂O₄, indicating the preferential growth of the constituent nanorod perpendicular to the (220) planes. The high quality HRTEM image clearly evidenced the high crystallinity of the as-obtained β-BaB₂O₄ microspheres.

Fig. 6 XRD patterns (a), SEM images (b–d, b₁ and c₁), TEM (e and e₁) and HRTEM (f and f₁) images as well as the size distribution histograms (b₂, c₂, d₂) of the calcined products originating from the phase conversion of the γ-BaB₂O₄ hollow microspheres at different temperatures for 3.0 h. Temperature (°C): a₁, b, b₁, b₂ – 500; a₂, c, c₁, c₂ – 600; a₃, d, d₁, d₂, e, e₁, f, f₁ – 700.

As confirmed above, with the temperature increasing from 500 °C to 600 °C and 700 °C, the average diameter of the calcined microspheres decreased from 13.7 µm to 13.4 µm, and to 11.2 µm, respectively. Notably, in contrast with that of the hydrothermally synthesized hierarchical γ-BaB₂O₄ hollow microspheres (Fig. 1(b) and (b₂)), the average size of the calcined microspheres tended to be smaller, due to the sintering during the high-temperature annealing. Taking the well-preserved morphology and slight change in the average size in the course of the phase conversion into consideration, both the hydrothermally synthesized γ-BaB₂O₄ hollow microspheres and calcined β-BaB₂O₄ microspheres exhibited excellent thermostability.

3.8 Hierarchical porous structure of the γ-BaB₂O₄ and β-BaB₂O₄ hollow microspheres

N₂ adsorption–desorption isotherms were recorded to examine the porous structures within the hierarchical γ-BaB₂O₄ and β-BaB₂O₄ hollow microspheres, as shown in Fig. 7. Both adsorption–desorption isotherms of the γ-BaB₂O₄ (Fig. 7(a)) and β-BaB₂O₄ (Fig. 7(b)) hollow microspheres demonstrated a type IV isotherm with H3-type hysteresis loops, revealing the existence of dominant slit pores and channels with a relatively uniform shape and size within the structures. Meanwhile, the occurrence of the narrow hysteresis loops at high relative pressures of P/P₀ = 0.8–1.0 (γ-BaB₂O₄, Fig. 7(a)) and P/P₀ = 0.9–1.0 (β-BaB₂O₄, Fig. 7(b)) indicated abundant mesopores and especially macropores contained within the hollow microspheres.^33,34 In particular, more constitutional macropores were present within the calcined loose β-BaB₂O₄ microspheres. The present hysteresis loops observed at high relative pressures were very similar to those of hierarchical porous Mg₂B₂O₅ superstructures⁶ and (Ni, Mg)₃Si₂O₅(OH)₄ solid-solution nanotubes.³⁵ The Barrett–Joyner–Halenda (BJH) pore size distribution (PSD) profiles of the γ-BaB₂O₄ (Fig. 7(a₁)) and β-BaB₂O₄ (Fig. 7(b₁)) hollow microspheres demonstrated pore diameters of 3–110 nm and 3–150 nm, respectively. This reconfirmed the presence of as-obtained γ-BaB₂O₄ (Fig. 7(a)) and β-BaB₂O₄ (Fig. 7(b)) hollow microspheres with abundant hierarchical porous structures, ranging from mesopores to macropores.

Fig. 7 Nitrogen adsorption–desorption isotherms (a and b) and the corresponding pore diameter distribution profiles (a₁ and b₁) of the γ-BaB₂O₄ hollow microspheres (a and a₁) hydrothermally synthesized at 180 °C for 12.0 h (i.e. Fig. 1(b)) and β-BaB₂O₄ hollow microspheres (b and b₁) calcined at 700 °C for 3.0 h (i.e. Fig. 6(d)).

The hierarchical γ-BaB₂O₄ hollow microspheres (Fig. 7(a)) exhibited a multipoint Brunauer–Emmett–Teller (BET) specific surface area of 46.25 m² g⁻¹, a pore volume of 0.306 cm³ g⁻¹, and an average pore diameter of 17.075 nm. In contrast, the calcined β-BaB₂O₄ hollow microspheres revealed a much smaller specific surface area (2.44 m² g⁻¹) and pore volume (0.025 cm³ g⁻¹), and an average pore diameter of 3.240 nm. The distinct decrease in the porosity characteristic parameters was probably attributed to the abrupt reduction or even disappearance of the multitudes of macropores (>50 nm) (Fig. 7(a₁) and (b₁)) on one hand and further growth of the constituent nanorods during the high-temperature annealing on the other hand.⁴⁸ The BET surface area of the present hierarchical γ-BaB₂O₄ hollow microspheres was somehow smaller than that of hierarchical porous MgBO₂(OH) microspheres (57.22 m² g⁻¹),⁶ but larger than those of porous Ca(BO₂)₂ microspheres (42.7 m² g⁻¹),¹² flower-like Bi₂MoO₆ hollow spheres (40.0 m² g⁻¹),³⁶ CoFe₂O₄ microspheres (32.3 m² g⁻¹),³⁷ and SrCO₃ submicron spheres (40.2 m² g⁻¹),³⁸ suggesting potential applications of the present hollow microspheres in catalysis, adsorption, etc.

3.9 FT-IR and UV-vis properties of the γ-BaB₂O₄ and β-BaB₂O₄ hollow microspheres

Fig. 8(a₁) and (b) show the FT-IR spectra of the hierarchical γ-BaB₂O₄ hollow microspheres hydrothermally synthesized at 180 °C for 12.0 h. The vibrational bands are concentrated in the wavenumber range of 1650–600 cm⁻¹. According to the FT-IR results of borates,³⁹ the band at 3440 cm⁻¹ was due to the O–H stretching of the absorbed water, the absorption peak at 1577 cm⁻¹ was owing to the H–O–H bending of the lattice water, the bands at 1385 cm⁻¹ and 1084 cm⁻¹ were attributed to the asymmetric stretching of B(3)–O, and the band at 923 cm⁻¹ corresponded to the symmetric stretching of B(3)–O. In addition, the out-of-plane bending of B(3)–O was located in the range of 730–635 cm⁻¹, and the band at 722 cm⁻¹ might be ascribed to the existence of the (B₃O₆)³⁻ group.^40,41 This further evidenced the high purity of the hierarchical γ-BaB₂O₄ hollow microspheres, similar to the previously reported results in the literature.⁴²

Fig. 8 FT-IR (a–c) and UV-vis (d) spectra of the γ-BaB₂O₄ hollow microspheres hydrothermally synthesized at 180 °C for 12.0 h (a₁, b, d₁ and d₃) and β-BaB₂O₄ hollow microspheres calcined at 700 °C for 3.0 h (a₂, c, d₂ and d₄).

Fig. 8(a₂) and (c) display the FT-IR spectra of the β-BaB₂O₄ hollow microspheres calcined at 700 °C for 3.0 h. The absorption band at 3450 cm⁻¹ might be due to the O–H stretching vibration of the absorbed water. The absorption peaks at 963 and 1425 cm⁻¹ were owing to the stretching vibration of extra-ring B–O bonds. The peak at 1247 cm⁻¹ was ascribed to the B–O stretching in the (BO₃)³⁻ unit, a component of the (B₃O₆)³⁻ ring.⁴³ The strong absorption band observed close to 700 cm⁻¹ corresponded to the bending vibrations of O–B–O. These FT-IR results of the β-BaB₂O₄ hollow microspheres were similar to those previously reported in literature.¹⁸

Fig. 8(d) demonstrates the UV-vis spectra of the hierarchical γ-BaB₂O₄ and β-BaB₂O₄ hollow microspheres dispersed in DI water. Obviously, both the hydrothermally synthesized γ-BaB₂O₄ hollow microspheres and the calcined β-BaB₂O₄ microspheres exhibited low absorption within the whole wavelength range of 200–900 nm (Fig. 8(d₁) and (d₂)). Meanwhile, the transmittance spectra (Fig. 8(d₃) and (d₄)) demonstrated that both the γ-BaB₂O₄ hollow microspheres and the calcined β-BaB₂O₄ microspheres exhibited good transparent characteristics from the ultraviolet to the visible regions. This was quite similar to the transparent nature of β-BaB₂O₄ nanospindles,¹⁸ β-BaB₂O₄ nanorods,¹⁹ β-BaB₂O₄ network-like nanostructures,⁴⁴ and magnesium borate nanowhiskers.⁴⁵

3.10 Adsorption performance of the hierarchical γ-BaB₂O₄ hollow microspheres

The hierarchical γ-BaB₂O₄ hollow microspheres were used as an adsorbent for the removal of Pb²⁺ ions from mimic waste water. Fig. 9(a) shows the variation of the adsorption rate of Pb²⁺ ions with time, where the initial concentration of the solution was 50 mg L⁻¹. It was obvious that the adsorption process proceeded quickly during the first 10 min, and equilibrium was achieved within 1.0 h. This indicated that the adsorption efficiency reached 78.14% within 10 min, and ultimately remained relatively constant at 80.83%. Meanwhile, the variation of the adsorption capacity at any instant (i.e. q_t) as a function of time is shown in Fig. 9(a₁). The high adsorption rate and adsorption capacity of Pb²⁺ onto the hierarchical γ-BaB₂O₄ hollow microspheres were probably attributed to the intrinsically and relatively large specific surface area as well as the surface alkalinity conditions. The maximum adsorption capacity (q_m) for Pb²⁺ was ca. 101.0 mg g⁻¹.

Fig. 9 Adsorption rate (a), adsorption capacity (a₁), pseudo-first-order model (b), pseudo-second-order model (c) and the composition of adsorbate after adsorption (d), using the hierarchical γ-BaB₂O₄ hollow microspheres as the adsorbent.

Compared with some other adsorbents for Pb²⁺ removal in the literature (Table 1), it was apparent that the hierarchical γ-BaB₂O₄ hollow microspheres exhibited higher adsorption abilities than urchin-like α-FeOOH hollow spheres,⁴⁶ organic silica hollow spheres⁴⁷ and BiOBr microspheres.⁴⁸ Notably, however, the q_m of the γ-BaB₂O₄ hollow microspheres was smaller than hierarchical porous MgO microrods,⁴⁹ porous Ca(BO₂)₂ microspheres,¹² and TiO₂ hollow spheres.⁵⁰ However, these hierarchical porous structures all originated from the hydrothermal synthesis of the corresponding metal-bearing hydrated precursors, followed by thermal conversion at high temperatures of 400–750 °C.^12,49,50 Taking the facile hydrothermal formation (no need for subsequent high-temperature calcination) and cost-effectiveness (cheaper than titania) into consideration, the current hierarchical γ-BaB₂O₄ hollow microspheres on the whole demonstrated satisfactory adsorption performance for the removal of Pb²⁺ ions from mimic waste water.

Table 1 Comparison of Pb²⁺ adsorption on hierarchical γ-BaB₂O₄ hollow microspheres and other adsorbents

Adsorbents	S BET	Pb^2+ adsorption capacity (mg g^−1)	Ref.
BiOBr microspheres	59.3	6.5	48
Organic silica hollow spheres	259.9	75.6	47
Urchin-like α-FeOOH hollow spheres	96.9	80	46
γ-BaB2O4 hollow microspheres	46.25	101	This work
Hierarchical porous MgO microrods	50.2	124.4	49
Porous Ca(BO2)2 microspheres	42.7	140.2	12
TiO2 hollow spheres	334	323	50

---

As is known, adsorption kinetics models describing the pollutant uptake rate during the adsorption process are significant characteristics in defining the adsorption efficiency. Thus, such models were employed so as to better understand the adsorption mechanism of Pb²⁺ ions on the hierarchical γ-BaB₂O₄ hollow microspheres. The linearized forms of the pseudo-first-order and pseudo-second-order kinetic models are expressed as follows:

where q_t (mg g⁻¹) is the adsorption capacity at time t, q_e (mg g⁻¹) is the equilibrium adsorption capacity, k₁ (min⁻¹) and k₂ (mg g⁻¹ min⁻¹) are rate constants of adsorption, and t (min) is the adsorption time. The pseudo-first-order and pseudo-second-order kinetic model fittings of the adsorption kinetics of Pb²⁺ are shown in Fig. 9(b) and (c). Apparently, the pseudo-second-order model displayed a correlation coefficient of R² = 0.9959, higher than that of the pseudo-first-order model (R² = 0.2567), and the calculated values of the adsorption capacities (q_e, cal) were very close to the experimental ones (q_e, exp), as listed in Table 2. This revealed that the pseudo-second-order model more accurately described the adsorption kinetics of Pb²⁺ ions on the γ-BaB₂O₄ hollow microspheres.

Table 2 Parameters of the kinetic models for the adsorption of Pb²⁺ on the hierarchical γ-BaB₂O₄ hollow microspheres

Adsorption kinetics	q e, exp (mg g^−1)	q e, cal (mg g^−1)	k 1 (min^−1) k2 (mg g^−1 min^−1)	R ^2
Pseudo-first-order models	101.0	5.458	k 1 = 0.006909	0.2567
Pseudo-second-order models	101.0	101.1	k 2 = 0.01224	0.9959

---

To further ascertain the satisfactory adsorption efficiency of Pb²⁺ on the present adsorbent, the XRD pattern of the adsorbent containing the adsorbate were recorded (Fig. 9(d)). As shown, the diffraction peaks could be assigned to γ-BaB₂O₄ (JCPDS no. 35-0181) and Pb₃(CO₃)₂(OH)₂ (JCPDS no. 13-0131). It was assumed that, during the adsorption process, partially dissolved CO₂ from the air reacted with Pb²⁺ and H₂O, producing the new phase of Pb₃(CO₃)₂(OH)₂. This suggested the formation of Pb₃(CO₃)₂(OH)₂ via the following reaction:

3Pb²⁺ + 2CO₂ + 4H₂O → Pb₃(CO₃)₂(OH)₂ + 6H⁺ (11)

Hence, the adsorption process conformed to physical adsorption of Pb²⁺ ions based on the hierarchical porous nature within the hierarchical γ-BaB₂O₄ microspheres and also the chemical adsorption of the Pb²⁺ ions and dissolved CO₂, which was similar to the adsorption of Pb²⁺ ions on hierarchical porous MgO microrods.⁴⁹

4. Conclusions

In summary, hierarchical γ-BaB₂O₄ hollow microspheres self-assembled from nanorods were synthesized by a facile EDTA-2Na assisted hydrothermal method (180 °C, 12.0 h), based on which β-BaB₂O₄ hollow microspheres with well-preserved spherical morphology and high crystallinity were obtained via the subsequent mild phase conversion (700 °C, 3.0 h). According to the effects of process parameters (e.g. reactant concentration, reaction time, temperature, and amount of the surfactant EDTA-2Na) on the hydrothermal product, a reasonable formation mechanism based on the EDTA-2Na assisted self-assembly was proposed. The formation mechanism of the γ-BaB₂O₄ hollow microspheres can be divided into three processes, including synthesis, hierarchical self-assembly and hollow structure formation. Both the hydrothermally synthesized hierarchical γ-BaB₂O₄ and calcined β-BaB₂O₄ hollow microspheres exhibited excellent thermostability. The hierarchical γ-BaB₂O₄ hollow microspheres possessed a specific surface area of 46.25 m² g⁻¹, a pore volume of 0.306 cm³ g⁻¹, and an average pore diameter of 17.075 nm. The γ-BaB₂O₄ and β-BaB₂O₄ hollow microspheres revealed a unique transparent nature from the ultraviolet to the visible regions. Meanwhile, the as-obtained γ-BaB₂O₄ hollow microspheres demonstrated a satisfactory ability to adsorb heavy metal Pb²⁺ ions from mimic waste water when employed as an adsorbent. The present work deepened the fundamental understanding and realistic practice of surfactant-assisted nanocrystal growth and hydrothermal self-assembly of NPs, and also expanded the future potential applications of the 3D hierarchical porous or hollow nanoarchitectures of metal borates in related fields.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21276141) and the State Key Laboratory of Chemical Engineering, China (no. SKL-ChE-15A03).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra12015f

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