Controlling the structure and morphology of zinc borate by adjusting the reaction temperature and pH value: formation mechanisms and luminescent properties

Abstract

Controlling the structure and morphology of micro/nanomaterials precisely enables us to manipulate their properties as desired. Therefore, understanding the complex growth mechanisms of micro/nanomaterials, and regulating and controlling their structure, morphology and related luminescent properties, are the focuses of chemistry and materials science. In this study, we hydrothermally synthesized three zinc borates with different structures and compositions, Zn(H₂O)B₂O₄·0.12H₂O, Zn₂(OH)BO₃, and H[Zn₆O₂(BO₃)₃], by adjusting the reaction temperature, and further prepared zinc borate, Zn(H₂O)B₂O₄·0.12H₂O, with three different morphologies by adjusting the pH of the reaction system. The possible growth mechanisms of these three zinc borate (Zn(H₂O)B₂O₄·0.12H₂O) morphologies are illustrated in detail. The luminescent properties of three different Tb³⁺-doped phosphors with different structures and Zn(H₂O)B₂O₄·0.12H₂O with three morphologies have also been investigated. Furthermore, the morphology–structure–property relationships have been discussed.

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Introduction

White light-emitting diodes (w-LEDs) have been regarded as the new generation of illumination sources due to numerous advantages, such as long lifetime, energy saving properties, and environmental friendliness. However, the commonly used w-LEDs, which are based on a combination of a blue InGaN LED chip and a yellow-emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce), have a low color rendering index and a high color temperature due to the lack of a red-emitting component. At present, researchers overcome the abovementioned problems by combining a near-UV LED chip with a white-emitting phosphor or blue, green and red emitting phosphors. Therefore, it is important to search for phosphors that could be excited efficiently by near-UV LEDs.^1–3 For a green emitting phosphor, Tb³⁺-doped phosphors are considered to be attractive candidates since the ⁵D₄–⁷F₅ transition of Tb³⁺ is around 540 nm. In principle, Tb³⁺, as a green activator, has a lower threshold pump power compared to Er³⁺ ions⁴ and a shorter decay time than that of Mn²⁺ ions.⁵ The luminescence properties of these materials are strongly dependent on their chemical compositions, phases, dimensions, and morphologies.⁶ Controlling the morphology and structure of micro/nanomaterials precisely enables us to manipulate their properties as desired. Therefore, understanding the complex growth mechanism of micro/nanomaterials, and regulating and controlling their structure, morphology and luminescent properties are the focuses of chemistry and materials science.

It is known that the structures and properties of inorganic materials are largely influenced by numerous factors during their assembly and/or post-treatment, such as the types of raw materials, concentration, temperature, and pH. Researchers have already paid special attention to the effects of pH on the assembly, morphology, and properties of inorganic materials.⁷ For example, Han, G. S. et al. controlled the morphology and phase of hydroxyapatite by adjusting the initial pH of precursors.⁸ Poongodi, S. et al. obtained nanoarray films of WO₃ with three different morphologies by adjusting the pH values of the electrolyte.⁹ Yin, X. H. et al. found that the pH value dramatically affected monodisperse YBO₃ microsphere formation.¹⁰ Kong, X. G. et al. demonstrated that the morphology of the layered K₂V₆O₁₆·nH₂O hexavanadate crystal could be easily controlled by adjusting the pH of the system.¹¹ Chu, L. H. et al. tuned the morphology of NiO by adjusting the pH of the precursor solutions with NH₃·H₂O via a hydrothermal process.¹² Song, J. L. et al. obtained a series of lead(II) borates by only adjusting the pH values of the reaction systems.¹³

Borates, as an important family of functional materials, have received much attention owing to their varied structural chemistry and wide application in photoluminescence,^14,15 nonlinear optical (NLO) devices,^16,17 flame retardant materials,¹⁸ electrochemical devices,¹⁹ photocatalysis,^20–22 and adsorbents.²³ Dozens of polyborates exist in aqueous borate solutions under common conditions. The existing forms and the interactions among these different polyborates mainly depend on pH, temperature, total boron concentration, and counter-ions in the solution.²⁴ Therefore, through tuning the reaction conditions, various forms of metal borate have been obtained with different chemical compositions and structures. An example is zinc borate, which can be categorized as hydrous zinc borate: Zn[B₃O₃(OH)₅]·H₂O, Zn[B₃O₄(OH)₃],²⁵ ZnO·2.2B₂O₃·3H₂O,²⁶ Zn₃B₁₀O₁₈·14H₂O,²⁷ 4ZnO·B₂O₃·H₂O,²⁸ Zn₈[(BO₃)₃O₂(OH)₃],²⁹ 3ZnO·3B₂O₃·3.5H₂O, 6ZnO·5B₂O₃·3H₂O,³⁰ H[Zn₆O₂(BO₃)₃],³¹ Zn₂B₆O₁₁·3H₂O,³² and Zn(H₂O)B₂O₄·0.12H₂O,³³ as well as anhydrous zinc borate: ZnB₄O₇,³⁴ ZnB₂O₄,³⁵ 4ZnO·3B₂O₃,³⁶ and Zn₃(BO₃)₂.³⁷ Tb³⁺-doped zinc borate phosphors have been studied for years. For instance, Li, J. et al. synthesized the novel phosphor, Zn(BO₂)₂:Tb, by a solid-state reaction.³⁸ Erdogmus, E. et al. synthesized Tb³⁺-doped Bi₂ZnB₂O₇ by a solid-state reaction.³⁹ Thulasiramudu, A. et al. obtained Eu³⁺/Tb³⁺-doped heavy metal oxide (HMO)-based zinc lead borate glasses by a solid-state reaction method.⁴⁰ Therefore, previous research on Tb³⁺-doped zinc borate generally involved preparation by a conventional solid-state reaction method, which required high temperatures as well as thorough grinding, and usually resulted in a poor morphology. To explore new metal borates with regular morphology and high photoluminescence (PL) efficiencies, we deem that studies on the reaction conditions of the borate systems are very interesting and important for understanding the structure–property relationships.

In this study, by controlling the reaction temperature and the pH of the prepared solutions, we synthesized three zinc borate structures (Zn(H₂O)B₂O₄·0.12H₂O, Zn₂(OH)BO₃, and H[Zn₆O₂(BO₃)₃]) and three Zn(H₂O)B₂O₄·0.12H₂O morphologies using a hydrothermal method with no surfactant or template. The morphology changes and structural evolution of zinc borate materials were studied. Furthermore, we also investigated the PL properties of these different structures and the different morphologies of Tb³⁺-doped zinc borate as well as the relationships between their morphology, crystal structures, and optical properties.

Experimental

Synthesis of samples

All of the reagents were of analytical grade and used directly without further purification.

For the synthesis of the three zinc borate structures, ZnSO₄·7H₂O (1.43 g), K₂B₄O₇·4H₂O (1.00 g), and NH₄HB₄O₇·3H₂O (2.50 g) were added into a 100 mL Teflon-lined stainless steel autoclave, and 60 mL H₂O was added with constant magnetic stirring. The pH of the mixed solution was adjusted to 9.5 with 3 M NaOH solution, and the solution was kept stirring for 30 min. After that, the Teflon-lined stainless steel autoclave was sealed and maintained at three temperature points (80 °C, 150 °C, and 220 °C) for 12 h, and allowed to cool to room temperature naturally. The white precipitates of Zn(H₂O)B₂O₄·0.12H₂O, Zn₂(OH)BO₃, and H[Zn₆O₂(BO₃)₃] were collected and washed several times with distilled water and absolute alcohol and then dried at 60 °C for 12 h.

For the synthesis of the three Zn(H₂O)B₂O₄·0.12H₂O morphologies, the procedures were the same as stated above (80 °C) except that the pH of the reaction system was changed from 8.0 to 9.0 and 9.5.

For the synthesis of Tb³⁺-doped zinc borate samples, the procedures were the same as stated above except that stoichiometric amounts of Tb(NO₃)₃·5H₂O were added.

Characterization

All samples were characterized by X-ray diffraction (XRD) (Rigaku D/max, operating at 40 kV and 30 mA, with a Cu target at a scanning rate of 8° min⁻¹, with a 2θ range from 10° to 50°), field emission scanning electron microscopy (FSEM) (SU-8020, Hitachi), and transmission electron microscopy (FTEM) (Tecnai G2 F20, FEI). The photoluminescence excitation and emission spectra were measured with an F-7000 (Hitachi) spectrophotometer equipped with a continuous 150 W xenon lamp at room temperature. For temperature-dependent experiments at 50–300 °C, the samples were placed in a small holder, the temperature of which was controlled by a heating device. The absolute photoluminescence quantum yield (QY) was measured with a Hamamatsu (C9920-02G) analytical instrument.

Results and discussion

Characterization and possible formation mechanism of samples

The influences of temperature on the composition and structure of zinc borate. The XRD patterns of samples obtained at various temperatures are shown in Fig. 1. It is observed that all the peaks of the sample prepared at 80 °C are in good agreement with those of the rhombohedral phase (space group: R3̄m) Zn(H₂O)B₂O₄·0.12H₂O (PDF, no. 97-028-1308). All the peaks of the sample prepared at 150 °C are in good agreement with those of the monoclinic phase (space group: P2₁) Zn₂(OH)BO₃ (PDF, no. 00-057-0146). All the peaks of the sample prepared at 220 °C are in good agreement with those of the trigonal phase (space group: R3̄c) H[Zn₆O₂(BO₃)₃] (PDF, no. 97-024-0473). No diffraction peaks of any other phases or impurities were detected in the three XRD patterns, indicating that all of the as-prepared samples are single-phase.

Fig. 1 XRD patterns and corresponding crystal structures of obtained zinc borates at different temperatures: (a) 80 °C; (b) 150 °C; (c) 220 °C.

The possible chemical reactions at different reaction temperatures can be formulated as follows:²⁸

B₄O₇²⁻ + 7H₂O → 4H₃BO₃ + 2OH⁻ (1)

HB₄O₇⁻ + 6H₂O → 4H₃BO₃ + OH⁻ (2)

NH₄⁺ + H₂O → NH₃·H₂O + H⁺ (3)

Zn²⁺ + 4OH⁻ → Zn(OH)₄²⁻ (4)

These results indicate that the reaction temperature of the current hydrothermal system has a significant role in the formation of the final products, which have different compositions and structures.

The reaction temperature drives the connection style of B–O, Zn–O and O–H, resulting in different compositions of the final products. When the reaction temperature is 80 °C, the final product, Zn(H₂O)B₂O₄·0.12H₂O, is constructed from the vertex linkages between the triangular B(1)O3 unit and the tetrahedral B(2)O4 and ZnO₄ units, as shown in the inset of Fig. 1a. The connectivity between the triangular B(1)O3 unit and the tetrahedral B(2)O4 creates a six-membered cyclic borate polyanion with the formula [B₁₂O₂₄]¹²⁻. The polyanion is formed by six B(2)O4 and six B(1)O3 triangles. Each B(2)O4 tetrahedron shares two of its corners externally with the B(1)O3 triangles. The addition of Zn²⁺ ions to the polyborate anion, [B₁₂O₂₄]¹²⁻, through Zn-μ₃O(1)–B bonds gives rise to the zinc borate cluster, Zn₆B₁₂O₂₄.³³ When the reaction temperature rises to 150 °C, the final product, Zn₂(OH)BO₃, has a chiral network structure built up from two independent types of ZnO₄ tetrahedra and BO₃ groups linked through common O vertices, as shown in the inset of Fig. 1b. The four oxygen atoms in both Zn(1)O4 and Zn(2)O4 are arranged in a very irregular tetrahedron. However, the unit cell of the compound contains crystallographically independent borate BO₃ groups, the zinc-centered tetrahedra sharing vertices to form a three-dimensional zinc oxide matrix, which consists of two types of –Zn–O–Zn– helical chains of distorted ZnO₄ tetrahedra running along the [100] and the [001] directions.⁴¹ When the reaction temperature rises to 220 °C, the final product, H[Zn₆O₂(BO₃)₃], is constructed of a triangular B(1)O3 unit and tetrahedral ZnO₄ units, as shown in the inset of Fig. 1c. Each O atom is surrounded by three cations, viz. three Zn atoms for atom O1, and two Zn and one B atom for atoms O2 and O3. Thus, a three-dimensional framework is formed. The main feature is the triple units of [ZnO₄] tetrahedra sharing one common O1 vertex that are connected by an equivalent unit over three corners (O2) and three [BO3] units to form a cage.³¹

FSEM images of the prepared zinc borate samples obtained at different temperatures are shown in Fig. 2. It is apparent that Zn(H₂O)B₂O₄·0.12H₂O displays hexahedral morphology with an edge of about 700 nm (Fig. 2a and a1). Zn₂(OH)BO₃ displays well-dispersed short belt-like morphology with a length of about 1.5 μm and a width of about 80 nm (Fig. 2b and b1). H[Zn₆O₂(BO₃)₃] displays well-dispersed polyhedral morphology with an average size of about 12 μm (Fig. 2c and c1).

Fig. 2 High and low resolution SEM images of obtained zinc borate at different temperatures: (a and a1) 80 °C; (b and b1) 150 °C; (c and c1) 220 °C.

HREM images of the prepared zinc borate samples obtained at different temperatures are shown in Fig. 3. The inter-planar spacing of the sample obtained at 80 °C is 2.368 Å, which is very close to the d value (2.374 Å) of the [0 4 2] faces in the Zn(H₂O)B₂O₄·0.12H₂O crystal. The inter-planar spacing of the sample obtained at 150 °C is 2.413 Å, which is very close to the d value (2.411 Å) of the [2 1 −1] faces in the Zn₂(OH)BO₃ crystal. The inter-planar spacing of the sample obtained at 220 °C is 2.458 Å, which is very close to the d value (2.454 Å) of the [0 1 −10] faces in the H[Zn₆O₂(BO₃)₃] crystal.

Fig. 3 High resolution TEM images of obtained zinc borates at different temperatures: (a) 80 °C; (b) 150 °C; (c) 220 °C.

The influences of pH on the morphology of Zn(H₂O)B₂O₄·0.12H₂O. XRD patterns of the prepared zinc borate samples with different pH values are displayed in Fig. 4. It is observed that all the peaks of the samples are in good agreement with those of the rhombohedral phase (space group: R3̄m) Zn(H₂O)B₂O₄·0.12H₂O (PDF, no. 97-028-1308). No diffraction peaks of any other phases or impurities were detected in this pattern, indicating that all of the as-prepared samples are single-phase.

Fig. 4 XRD patterns of obtained zinc borate at different pH values: (a) pH = 8.0; (b) pH = 9.0; (c) pH = 9.5.

FSEM images of the prepared Zn(H₂O)B₂O₄·0.12H₂O samples obtained at different pH values are shown in Fig. 5. It is apparent that the morphologies of the Zn(H₂O)B₂O₄·0.12H₂O samples are closely related to the pH values of the precursor solution during the synthesis process. The sample synthesized at pH 8.0 displays well-dispersed octahedral morphology with an edge of about 1.5 μm (Fig. 5a and a1). When the pH of the precursor solution rises to 9.0, the obtained sample displays distorted octahedral morphology with a long edge of about 600 nm (Fig. 5b and b1). When the pH of the precursor solution rises to 9.5, the obtained sample displays hexahedral morphology with an edge of about 700 nm (Fig. 5c and c1).

Fig. 5 High and low resolution SEM images of obtained zinc borate at different pH values: (a and a1) pH = 8.0; (b and b1) pH = 9.0; (c and c1) pH = 9.5.

The morphology evolution of Zn(H₂O)B₂O₄·0.12H₂O at the pH of the precursor solution indicates the significant influence of the pH on the morphology of the products. However, using NaOH to increase the system pH increases the amount of Na⁺ ions, which can also affect the morphology of the materials.⁴² Therefore, we also added different amounts of NaNO₃, Na₂SO₄ and K₂SO₄ when the system pH had been adjusted to 9.5. However, the phase and morphology of the obtained samples were not changed significantly when the abovementioned salts were added. Therefore, we deem that OH⁻ plays a critical role in changing the morphology of Zn(H₂O)B₂O₄·0.12H₂O. Moreover, when NH₃·H₂O and KOH were used to adjust the pH of the system, we obtained similar results to those with NaOH.

A possible formation mechanism for the Zn(H₂O)B₂O₄·0.12H₂O samples with various morphologies is proposed, as shown in Scheme 1. During the hydrothermal process, the ions in aqueous solution initially reacted quickly, precipitating amorphous particles. When the pH value of the precursor solution was weak alkaline, the lower concentration of OH⁻ ions resulted in the formation of Zn(OH)₄²⁻, leading to slow growth of the crystal and weak aggregation of the crystal nuclei. Furthermore, Zn(H₂O)B₂O₄·0.12H₂O crystals were formed through the intermolecular reaction. The surface of Zn(H₂O)B₂O₄·0.12H₂O can attach a number of OH⁻ groups. Under non-equilibrium conditions, the shape of the crystals varies and strongly depends on kinetic parameters, such as the crystal growth rate during the nucleation and growth of crystals.⁴³ At pH 8.00, an octahedral morphology of Zn(H₂O)B₂O₄·0.12H₂O with exposed [1 1 1] faces can be obtained. Thermodynamic analysis indicated that the supersaturation of the growth species (the concentration of OH⁻ in this system) might determine the surface energy of exposed faces during crystal growth. According to the Gibbs–Curie–Wulff theorem,²⁸ the growth rates on different surface facets are dominated by the surface energy. When a particle grows, the facets tend toward low-energy planes to minimize the surface energy. The energy of [1 0 0] faces is lower than that of [1 1 1] faces. When the pH rises to 9.00, the solution generates more OH⁻, resulting in a higher growth rate of Zn(OH)₄²⁻ nuclei. Therefore, a disordered octahedral morphology of Zn(H₂O)B₂O₄·0.12H₂O with exposed [1 1 1] and [1 0 0] faces can be obtained. With a further increase in the pH of the precursor solution to 9.50, a higher content of OH⁻ leads to a higher growth rate of Zn(OH)₄²⁻ nuclei. Therefore, favored by the higher growth rate and the spontaneous energy-minimizing process removing surface energy associated with unsatisfied bonds, the resulting Zn(OH)₄²⁻ nuclei gradually self-organize and react further. Hence, a hexahedral morphology of Zn(H₂O)B₂O₄·0.12H₂O with exposed [1 0 0] faces can be obtained. From the octahedral, to the disordered octahedral, to the hexahedral morphology of Zn(H₂O)B₂O₄·0.12H₂O, we found that when the up-down symmetrical points of the octahedra and their linked faces grew along a parallel line, disordered octahedral morphology could be obtained. With further growth along the parallel line, a hexahedral morphology of Zn(H₂O)B₂O₄·0.12H₂O could be obtained.

Scheme 1 Possible growth mechanisms of the obtained zinc borate at varied pH values.

Photoluminescence properties of Tb³⁺-doped zinc borates

The effect of doped Tb³⁺ concentration on the PL intensity of Zn(H₂O)B₂O₄·0.12H₂O:xTb³⁺. The PLE and PL spectra under UV excitation for Zn(H₂O)B₂O₄·0.12H₂O:xTb³⁺ samples (x = 0.001, 0.005, 0.01, 0.03, 0.05, 0.07 and 0.10) were measured at room temperature. The excitation spectra of Zn(H₂O)B₂O₄·0.12H₂O:xTb³⁺ (x = 0.001, 0.005, 0.01, 0.03, 0.05, 0.07 and 0.10), taken at an emission wavelength of 545 nm, are shown in Fig. 6 (left). No remarkable differences in the spectroscopic characteristics were observed in the excitation spectra when different concentrations of Tb³⁺ ions were doped. Below 300 nm, the sharp band at 225 nm corresponds to the spin-allowed 4f⁸ → 4f⁷5d¹ (⁷F₆ → ⁷D) transition of Tb³⁺, and the band at 272 nm corresponds to the spin-forbidden 4f⁸ → 4f⁷5d¹ (⁷F₆ → ⁹D) transition of Tb³⁺.⁴⁴ The weak bands at 319, 339, 352, 368 and 379 nm correspond to the f–f transitions of Tb³⁺ in the host lattice. For all the Tb³⁺ concentrations, the shapes of the excitation spectra are essentially the same except for the intensity at different activator concentrations.

Fig. 6 PL excitation (left) and emission spectra (right) of Zn(H₂O)B₂O₄·0.12H₂O:xTb³⁺ synthesised at different concentrations: (a) x = 0.001, (b) x = 0.005, (c) x = 0.01, (d) x = 0.03, (e) x = 0.05, (f) x = 0.07, (g) x = 0.10.

The emission spectra of Zn(H₂O)B₂O₄·0.12H₂O:xTb³⁺ (x = 0.001, 0.005, 0.01, 0.03, 0.05, 0.07 and 0.10) at room temperature (RT) are shown in Fig. 6 (right), which exhibit the same spectroscopic features for different concentrations of Tb³⁺ ions in the samples. The emission spectra are composed of one hump band and four sharp bands. The hump band centered at about 380 nm can be attributed to zinc borate host-related emission, which is called ‘‘band-gap’’ fluorescence.⁴¹ The four sharp bands with maxima at about 491, 545, 587, and 623 nm are due to the transitions from the excitation state ⁵D₄ to the ground states ⁷F_J (J = 6, 5, 4, 3) of Tb³⁺ in the host lattice. Among these transitions, the green emission located at 545 nm (⁵D₄ → ⁷F₅) is the strongest peak because it is a magnetic dipole allowed with the Δ_J = ±1 rule. Therefore, we can deduce that Tb³⁺ is located at a low symmetry site of Zn in the Zn(H₂O)B₂O₄·0.12H₂O host lattices.¹ For all Tb³⁺ concentrations, the shapes of the emission spectra are essentially the same except for the intensity at different activator concentrations. As the concentration of Tb³⁺ increased gradually, the emission associated with Tb³⁺ also increased up to 7 mol% doping concentration and then decreased because of concentration quenching. The concentration quenching effect might be due to the enhanced excitation migration or to the activator ions that are coupled or coagulated and migrate to the quenching center.¹

The temperature-dependent behavior of the PL emission spectra (Fig. 7) shows considerable stability for Zn(H₂O)B₂O₄·0.12H₂O:0.07Tb³⁺ in the temperature range 50–300 °C. As the temperature increases from 50 °C to 250 °C, the shape and PL intensity of the emission spectra does not change significantly, which shows that the thermal stability of the optimized-composition sample is good. A further increase in the temperature above 250 °C (275 °C, 300 °C) will increase the non-radiative transition probability and the PL intensity shows thermal quenching.⁴⁵ Moreover, the quantum yield of Zn(H₂O)B₂O₄·0.12H₂O:0.07Tb³⁺ under 352 nm excitation is about 10.2% at room temperature.

Fig. 7 PL emission spectra of Zn(H₂O)B₂O₄·0.12H₂O:0.07Tb³⁺ measured at different temperatures.

The effect of morphology on the PL intensity of Zn(H₂O)B₂O₄·0.12H₂O:xTb³⁺. The PLE and PL spectra under UV excitation for Zn(H₂O)B₂O₄·0.12H₂O:0.07Tb³⁺ samples synthesized at different pH values were measured at room temperature. The excitation spectra of Zn(H₂O)B₂O₄·0.12H₂O:0.07Tb³⁺, taken at an emission wavelength of 545 nm, are shown in Fig. 8 (left). No remarkable differences in the spectroscopic characteristics of the samples were observed in the excitation spectra when synthesized at different pH values. Below 300 nm, the sharp band at 226 nm corresponds to the spin-allowed 4f⁸–4f⁷5d¹ (⁷F₆–⁷D) transition of Tb³⁺, and the band at 272 nm is related to the spin-forbidden 4f⁸–4f⁷5d¹ (⁷F₆–⁹D) transition of Tb³⁺. The weak excitation bands at 319, 340, 352, 369 and 378 nm correspond to the f–f transitions of Tb³⁺ in the host lattice. For all the different pH values, the shapes of the excitation spectra are essentially the same except for the intensity at different pH values. The PL excitation intensities of these zinc borates prepared at different pH values are significantly different, as shown in Fig. 8, because the excitation intensities are in accordance with the emission intensities, which result from their morphology and exposed crystal facets.

Fig. 8 PL excitation (left) and emission spectra (right) of Zn(H₂O)B₂O₄·0.12H₂O:0.07Tb³⁺ at different pH values: (a) pH = 8.0, (b) pH = 9.0, (c) pH = 9.5.

The emission spectra of Zn(H₂O)B₂O₄·0.12H₂O:0.07Tb³⁺ at room temperature (RT) are shown in Fig. 8 (right), which exhibit the same spectroscopic features for different morphologies of the samples. The shapes of the emission spectra are essentially the same except for the intensity at different pH values. The emission spectra are composed of one hump band and four sharp bands. The hump band centered at about 385 nm can be attributed to zinc borate host-related emission, which is called “band-gap” fluorescence. The four sharp bands with maxima at about 490, 544, 586, and 623 nm are due to the transitions from the excitation state ⁵D₄ to the ground states ⁷F_J (J = 6, 5, 4, 3) of Tb³⁺ in the host lattice. The sample prepared at pH = 9.5 (hexahedral morphology) exhibited the strongest intensity. It is widely accepted that the luminescent properties of phosphors are strongly correlated with the morphology, size, crystallinity, the center of luminescence and the density of defects.⁴⁶ The hexahedral morphology of Zn(H₂O)B₂O₄·0.12H₂O:0.07Tb³⁺ has a shorter edge with a larger specific surface area than the octahedral morphology, which will result in a high concentration of luminescence centers, causing enhancement of the emission intensity.⁴⁷ Furthermore, the [100] faces of the hexahedral morphology possess a lower energy than the combined [1 1 1] and [1 0 0] faces of the disordered octahedral morphology and the [1 1 1] faces of the octahedral morphology of Zn(H₂O)B₂O₄·0.12H₂O:0.07Tb³⁺, which results in a lower trapping effect with respect to phonon emission and transition. Therefore, the hexahedral morphology of Zn(H₂O)B₂O₄·0.12H₂O:0.07Tb³⁺ possesses the highest emission intensity.

The effect of structure on the PL intensity of Tb³⁺-doped zinc borate. The PLE and PL spectra under UV excitation for different structures of zinc borates, (Zn(H₂O)B₂O₄·0.12H₂O:0.07Tb, Zn₂(OH)BO₃:0.07Tb, and H[Zn₆O₂(BO₃)₃]:0.07Tb) were measured at room temperature. The excitation spectra of the different structures of zinc borates taken at an emission wavelength of 545 nm are shown in Fig. 9 (left). No remarkable differences in the spectroscopic characteristics of the samples were observed in the excitation spectra when synthesized at different temperatures. Below 300 nm, the sharp band at 226 nm corresponds to the spin-allowed 4f⁸–4f⁷5d¹ (⁷F₆–⁷D) transition of Tb³⁺, and the band at 272 nm is related to the spin-forbidden 4f⁸–4f⁷5d¹ (⁷F₆–⁹D) transition of Tb³⁺. The weak excitation bands at 319, 340, 352, 369 and 378 nm correspond to the f–f transitions of Tb³⁺ in the host lattice. The peak position of the spin-allowed 4f⁸ → 4f⁷5d¹ (⁷F₆ → ⁷D) transition of Tb³⁺ is blue-shifted in the three zinc borate structures. The blue shift of the excitation band, along with the decrease in the full width at half maximum (fwhm) of the excitation bands could be explained as a nephelauxetic effect. The nephelauxetic effect will result in a shift of the energy barycenter of the 4f levels relative to the free ion value.³ When Zn²⁺ was substituted by Tb³⁺ in the abovementioned three structures, the nephelauxetic effect could be different. Furthermore, due to the difference in the distance between the central metal (Zn) ion and its ligands (O) in the abovementioned three zinc borate structures (1.978 Å for Zn(H₂O)B₂O₄·0.12H₂O, 1.963 Å for Zn₂(OH)BO₃, and 1.955 Å for H[Zn₆O₂(BO₃)₃]), when some Tb³⁺ ions replaced Zn²⁺ ions, the center of the 4f⁸ → 4f⁷5d¹ (⁷F₆ → ⁷D) transition of Tb³⁺ could be shifted.⁴⁵

Fig. 9 PL excitation (left) and emission spectra (right) of different structures of zinc borateahcas: (a) Zn(H₂O)B₂O₄·0.12H₂O:0.07Tb, (b) Zn₂(OH)BO₃:0.07Tb, (c) H[Zn₆O₂(BO₃)₃]:0.07Tb.

The emission spectra of the abovementioned three zinc borate phosphors at room temperature (RT) are shown in Fig. 9 (right), which exhibit the same spectroscopic features for different structures of Tb³⁺ ions in the samples. For all the different structures of Tb³⁺ ions in the samples, the shapes of the emission spectra are generally the same except for the intensities. The emission spectra are composed of one hump band and four sharp bands. The hump band centered at about 385 nm can be attributed to zinc borate host-related emission, which is called “band-gap” fluorescence. The four sharp bands with maxima at about 490, 544, 586, and 623 nm are due to the transitions from the excitation state ⁵D₄ to the ground states ⁷F_J (J = 6, 5, 4, 3) of Tb³⁺ in the host lattice. The Zn(H₂O)B₂O₄·0.12H₂O:0.07Tb sample exhibited the strongest intensity. Furthermore, the band gap emission in H[Zn₆O₂(BO₃)₃]:0.07Tb was obviously higher than the characteristic Tb³⁺ emission. Upon VUV excitation, it is considered that most of the incident photons are absorbed by the host crystal moving electrons from the valence band toward the conduction band, subsequently producing either self-trapped excitons (STEs) or free electron–hole pairs. This emission band most likely arises from the recombination of STEs that are associated with band-gap excitations or molecular transitions within the BO₃³⁻ group.⁵ The top of the valence band comprises hybridized O 2p and B 2p-like states, whereas the bottom of the conduction band may contain metallic states. The key to the band gap of borates has been shown to be the elimination of nonbonding orbitals from oxygen atoms, the so-called dangling bonds in the valence band.⁴⁸ The sample (H[Zn₆O₂(BO₃)₃]:0.07Tb) obtained at the highest temperature, possesses the largest volume and the smallest specific surface area. Therefore, it owns the fewest dangling bonds and the strongest band gap emission. The various structures will produce different crystal field environments for the doped activators, which further affects the luminescence properties of the as-prepared zinc borate phosphors.

In the present case, by changing the reaction temperature, we can obtain different compositions and structures of zinc borate, which will affect the crystal field environment around the doped Tb³⁺ ions, and thus the color-tunable emission can be controlled through changing the structures of the zinc borate. The corresponding variation in the CIE chromaticity coordinates under a 225 nm UV lamp is demonstrated and summarized in Fig. 10. The color tone can be adjusted from green (0.230, 0.454), to light green (0.223, 434), to light blue (0.229, 0.342) in the zinc borate phosphors by varying the chemical composition of the solid solution phosphor.

Fig. 10 The corresponding variation in the CIE chromaticity coordinates under a 225 nm UV lamp: (a) Zn(H₂O)B₂O₄·0.12H₂O:0.07Tb, (b) Zn₂(OH)BO₃:0.07Tb, (c) H[Zn₆O₂(BO₃)₃]:0.07Tb.

Conclusions

Three different structures of zinc borates for Zn(H₂O)B₂O₄·0.12H₂O, Zn₂(OH)BO₃, and H[Zn₆O₂(BO₃)₃], and three different morphologies of Zn(H₂O)B₂O₄·0.12H₂O have been prepared through a hydrothermal method. The reaction-temperature-driven structural changes of zinc borate were discussed in detail. The reason why the pH value affected the morphology was also discussed in detail. The photoluminescence (PL) analysis results manifested that the 7% doped Zn(H₂O)B₂O₄·0.12H₂O sample possesses the highest intensity. Furthermore, the PL results indicated that the hexahedral morphology possesses the highest intensity. Moreover, the structure–property relationships of zinc borate have also been discussed.

Acknowledgements

This study is supported by the National Natural Science Foundation of China (No. 21573142).

Notes and references

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