Orange/cyan emissive sensors of Sb³⁺ for probing water via reversible phase transformation in rare-earth-based perovskite crystals

Abstract

A facile strategy for water probing with visible perception is particularly important for water transport, ecosystem sustainability, and the ocean industry. Here, we have developed a simple technology to probe water content by using the response of the orange/cyan luminescence color of Sb³⁺ emitted from the two novel rare-earth-based perovskite crystals Rb₂ScCl₅·H₂O and Rb₃ScCl₆. The reversible transformation between Rb₂ScCl₅·H₂O and Rb₃ScCl₆ accompanied by the significant visible perception of the change of the luminescence color can be realized through the dehydration/hydration process in air. In addition to the favorable sensitivity to water, an in-depth study shows that the highly efficient photoluminescence (PL) of Sb³⁺ in both Rb₂ScCl₅·H₂O and Rb₃ScCl₆ host matrixes arises from the self-trapped excitons (STEs) rather than the 5s²–5s¹5p¹ transition of Sb³⁺ according to the density functional theory (DFT) calculation. It can be anticipated that this study will provide new insights into the luminescence mechanism of Sb³⁺ in perovskite crystals and a potential application in a sensitive sensor for probing water content.

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1 Introduction

A conventional moisture indicator for the validity of a silica gel dryer often relies on the hydration/dehydration process of CoCl₂/[CoCl(H₂O)₅]Cl·H₂O accompanied by a color change (blue/pink). The reversible switch or transformation in the PL color of luminescence materials responding to water would be an alternative visual sensor for probing water content in the manufacture and transport of chemical industrial products and anti-counterfeiting. The reversible transition between luminescent CsPbBr₃ and non-luminescent CsPb₂Br₅ has been achieved upon the exposure/removal of water by loading CsPbBr₃ NCs into a porous matrix.¹ The emissive light of Sb³⁺ can be sensitively switched between red/yellow in zero-dimensional (0D) halide single crystals (PPZ)₂SbCl₇·5H₂O (PPZ = 1-phenylpiperazine) and Cs₂InBr₅·H₂O:Sb³⁺ ascribed to the insertion/removal of the water molecules into/from the matrixes.^2–4 Environmentally friendly alternatives free of Pb and organics, and possessing high PLQYs and detection sensitivity, are still in demand in the growing field for application in sensors and anti-counterfeiting.

Recently, emerging luminescence materials based on Sb³⁺ doped all-inorganic halide perovskite crystals have become outstanding candidates for promising application in lighting and displays due to their advantages of excellent optical properties including high photoluminescence efficiency and a broad emitting band covering from the visible to near IR region.^5–23 The introduction of Sb³⁺ dopants has led to a significant increase in the intrinsic PL efficiency of the all-inorganic halide perovskite crystals.^7,8 It has been well documented that the Sb³⁺ ion possesses the ns² electronic configuration and its excited state is separated into the singlet state ¹P₁ and triplet state (³P_n, n = 1, 2, 3). The excitation transitions from the ground ¹S₀ to singlet state ¹P₁ are allowed and one of the triplet states ³P₁ is partially allowed, whereas the transitions from ¹S₀ to ³P₂ and ³P₀ are forbidden.^18–23 However, it is usually hard to make a decision on whether the PL induced by Sb³⁺ doping is attributed to the 5s²–5s¹5p¹ transition of Sb³⁺ or the STEs of the halide perovskite matrix with new trapping states produced by Sb³⁺ because they both are similar in PL spectra, such as broadband emission with a large Stokes shift. It has been reported that the experimental and computational studies reveal that the origin of red PL in Cs₂InBr₅·H₂O:Sb³⁺ is attributed to STEs induced by a structural deformation, whereas the green emission of Cs₃InCl₆:Sb³⁺ NCs is ascribed to the inter-configurational ³P₁ → ¹S₀ transition of Sb³⁺.^4,13 Even though substantial efforts have been devoted to improving the PL efficiency of Sb³⁺-doped perovskites and the PLQYs are up to 95%, the demand for an in-depth understanding of the luminescence mechanism is booming. Therefore, further evidence should be provided to confirm the underneath mechanism because both the STEs and ³P₁ → ¹S₀  transition of Sb³⁺ are actually correlated with the strong Jahn–Teller effect resulting from the introduction of exotic ions. The intense broadband cyan-green and orange-yellow photoluminescence originating from [SbCl₆]³⁻ in (Rb/Cs)₃InCl₆:Sb³⁺ and [SbCl₅(H₂O)]²⁻ in Cs₂InCl₅·H₂O:Sb³⁺ has been identified, respectively.^5,6 The corresponding emission colors of Cs₂InX₅·H₂O:Sb³⁺ (X = Cl, Br, I) can be expanded into the orange-red region through the compositional substitution of various halides.^4,12 A reversible cyan/yellow emission switch is realized with Sb³⁺ in the transformation of the crystals of A₃InX₆ and A₂InX₅·H₂O (A = Cs, Rb; X = Cl, Br) due to the dehydration/hydration process.^5,13,15 Therefore, Sb³⁺ ions in perovskite crystals exhibit distinctly different PL, which can be switched by the addition/removal of H₂O into/from the ligands of the Sb³⁺ octahedron.

Herein, two novel Pb-free perovskite crystals Rb₂ScCl₅·H₂O and Rb₃ScCl₆ with rare-earth Sc³⁺ as central ions are identified for the first time, which have similar structures to those of the established Cs₂ScCl₅·H₂O and Cs₃ScCl₆ crystals, respectively.²⁴ The combination of the systematic experimental and computational results validates that the PL spectra of Sb³⁺ in Rb₂ScCl₅·H₂O and Rb₃ScCl₆ with PLQYs up to 99% probably originate from STEs rather than the 5s²–5s¹5p¹ transition of Sb³⁺ because the 5p states of Sb³⁺ are far higher than the bottom of the conduction band. The sensitive switch of the orange/cyan luminescence color of Sb³⁺ responding to water is observed, which is ascribed to the reversible transformation between the matrixes of Rb₂ScCl₅·H₂O and Rb₃ScCl₆. The results of this work could offer an in-depth understanding of the luminescence mechanism of Sb³⁺ in perovskite crystals and provide new possibilities in water sensors, where both high PLQYs and sensitive water response are strongly desired.

2 Experimental

2.1 Raw materials

For the experiments, the raw material RbCl (99.9%) was purchased from Meryer (Shanghai, China). Sc₂O₃ (99.9%), and Sb₂O₃ (99.9%), hydrochloric acid HCl (36.5 wt%), and ethyl alcohol were purchased from Aladdin (Shanghai, China). All the starting materials for synthesis were purchased commercially and used as received without further purification.

2.2 Synthesis of Sb³⁺ doped Rb₂ScCl₅·H₂O single crystals via hydrothermal strategy

For the undoped Rb₂ScCl₅·H₂O single crystals, 2 mmol of RbCl, 0.5 mmol of Sc₂O₃, and 3 mL of hydrochloric acid were mixed in a 15 mL Teflon liner. The obtained solution was heated in a stainless steel Parr autoclave at 150 °C for 12 h, and then naturally cooled to room temperature. The supernatant solution was removed by tilting the Teflon liner and the precipitated crystals were washed three times with ethyl alcohol and dried at 60 °C for 12 h. The similar experimental procedures were carried out to obtain the Sb³⁺ doped Rb₂ScCl₅·H₂O single crystals by replacing x mmol of Sc₂O₃ with Sb₂O₃ (x = 0.0025, 0.005, 0.01, 0.015, 0.025, 0.04, 0.05, 0.075, 0.1) by keeping the stoichiometric ratios.

2.3 A reversible transformation process between Sb³⁺ doped Rb₂ScCl₅·H₂O and Rb₃ScCl₆ crystals induced by water

In a typical process, 50 mg of the as-prepared Rb₂ScCl₅·H₂O:Sb³⁺ single crystals with orange emission were ground into powder and then heated at different temperatures (100, 150, 200, 220, 230, 240, and 250 °C) for 2 h in an oven. The sintered products with cyan emission were characterized by PL photographs, spectra, and XRD. The dominant component in the final products sintered at 250 °C was confirmed to be Rb₃ScCl₆:Sb³⁺. For the returning transformation procedure, 3 mg of Rb₃ScCl₆:Sb³⁺ powder was separately put in air with/without a glass lid. After 60 min, the Rb₃ScCl₆:Sb³⁺ powder exposed to air changed to be Rb₂ScCl₅·H₂O:Sb³⁺ completely while that with an insulating lid maintained the insensible change. The dependence of time required for the transformation procedure on the relative humidity and the contract area was investigated.

2.4 Fabrication and measurement of a WLED with Rb₂ScCl₅·H₂O:Sb³⁺

A typical WLED was fabricated using a 360 nm chip, a blue phosphor BaMgAl₁₀O₁₇:Eu²⁺ (BAM:Eu), a green phosphor BaSrSiO₄:Eu²⁺ (BSSO:Eu), and the as-prepared orange phosphor Rb₂ScCl₅·H₂O:Sb³⁺. The three phosphors were thoroughly mixed with epoxy resin and coated on the surface of the LED chips. The devices were solidified at 130 °C for 1 h to produce a solid-state WLED.

3 Results and discussion

Single-crystal X-ray diffraction (SCXRD) has been used to determine that the novel single crystal Rb₂ScCl₅·H₂O adopts the orthorhombic space group Pnma with unit cell parameters of a = 14.003 Å, b = 10.045 Å, c = 7.246 Å, α = β = γ = 90° (Fig. 1a and Table S1†). The detailed crystal structure parameters of Rb₂ScCl₅·H₂O obtained from the single crystal analysis have been compared to those of Cs₂ScCl₅·H₂O (Table S2†). In Rb₂ScCl₅·H₂O crystals, each rare earth Sc³⁺ ion in a deformed octahedron is coordinated by five Cl⁻ ions and one H₂O. The bond lengths of Sc–Cl in Rb₂ScCl₅·H₂O range from 2.4510(14) to 2.4873(15) Å, which are longer than those from 2.4589(9) to 2.4845(5) Å in Cs₂ScCl₅·H₂O. Besides, the angle of Cl–Sc–O is 86.591(3)° in Rb₂ScCl₅·H₂O, which is larger than that of 85.498(17)° in Cs₂ScCl₅·H₂O.²⁴ The results mean the lattice of Rb₂ScCl₅·H₂O is more easily deformed by doping Sb³⁺ ions and is conducive to producing STEs.²⁵

Fig. 1 (a and b) XRD patterns of Rb₂ScCl₅·H₂O:x%Sb³⁺ (x = 0, 0.5, 1, 2, 3, 5, 8, 10, 15, and 20) compared to the corresponding simulated result of Rb₂ScCl₅·H₂O. (c) EDS pattern, (d) SAED, (e) HRTEM, and (f–k) mapping images of the elements in Rb₂ScCl₅·H₂O:Sb³⁺.

In order to produce PL in Rb₂ScCl₅·H₂O, parts of Sc³⁺ ions are substituted by equivalent Sb³⁺ ions. The powder X-ray diffraction (PXRD) measurements are carried out to analyze the phase purity after doping Sb³⁺ in different amounts (Fig. 1a). The results show that all the diffraction peaks of the crystals with the doping concentration of Sb³⁺ in 0.5–15 mol% of Sc³⁺ are consistent with simulated orthorhombic Rb₂ScCl₅·H₂O, indicating the successful introduction of Sb³⁺ ions into the matrix of Rb₂ScCl₅·H₂O. The sharp diffraction peaks are indicative of the high crystallinity of these crystals doped with various concentrations of Sb³⁺. Until the doping ratio of Sb³⁺ increases to 20 mol%, an extra diffraction peak can be observed at 27°, which is ascribed to the impurity of the raw material RbCl. With the Sb³⁺ concentration increasing, the XRD diffraction peaks shift to the lower angles (Fig. 1b), demonstrating the expanding of the crystal lattice because the ionic radius of Sb³⁺ (0.92 Å) is larger than that of Sc³⁺ (0.81 Å). Moreover, the EDS pattern of Rb₂ScCl₅·H₂O:Sb³⁺ with 2 mol% Sb³⁺ dopants confirms the presence of the constituent elements (Rb, Sc, Cl, O, and Sb) without any impurities (Fig. 1c). The selected area electron diffraction (SAED) pattern exhibits two distinct hexagonal diffraction patterns, which correspond to the interplanar distances of the (3 0 1) and (2 5 0) planes of Rb₂ScCl₅·H₂O (Fig. 1d). The HRTEM image shows clear lattice spacing of 0.238 and 0.163 nm, which are in good agreement with the (1 0 3) and (2 6 0) planes of the orthorhombic Rb₂ScCl₅·H₂O crystal, respectively (Fig. 1e). The elemental mapping images are used to observe the distribution uniformity of each composing element in Rb₂ScCl₅·H₂O:Sb³⁺ (Fig. 1f–k). It is obvious that the Rb, Sc, Cl, O, and Sb elements are evenly distributed in the whole crystals. XPS measurements were performed to further prove the composition and analyze the chemical states of the elements in the Rb₂ScCl₅·H₂O:Sb³⁺ crystal (Fig. S1†). The binding energies at 110.98, 109.55, 407.58, 403.05, 199.70, 198.28, 532.18, and 540.31 eV match well with the corresponding spin orbitals of Rb 3d_3/2, Rb 3d_5/2, Sc 2p_1/2, Sc 2p_3/2, Cl 2p_1/2, Cl 2p_3/2, O 1s and Sb 3d, respectively.^26–28

The PL excitation and emission spectra are illustrated to explore the optical properties of Rb₂ScCl₅·H₂O:x%Sb³⁺ (x = 0, 0.5, 1, 2, 3, 5, 8, 10, 15, and 20) crystals (Fig. 2a and b). Pure Rb₂ScCl₅·H₂O is non-emissive under ultraviolet (UV) light. For the Rb₂ScCl₅·H₂O:xSb³⁺ crystals, a single excitation band at 340 nm and a strong broad orange PL emission band at 600 nm with a full width at half maximum (FWHM) of 141 nm have been observed, respectively. As the content of Sb³⁺ increases from 0.5 to 2 mol% of Sb³⁺, the PL intensity reaches a maximum. The concentration quenching effect occurs after the concentration of Sb³⁺ increases further, which leads to the decrease of the PL intensity. Furthermore, similar PLE spectra manifest that the excited states and dynamics are independent of the content of Sb³⁺. The dependence of the emission wavelength and intensity of Rb₂ScCl₅·H₂O:xSb³⁺ on the concentration of Sb³⁺ has been explored (Fig. 2c). A slightly blue shift in the emission spectra of Rb₂ScCl₅·H₂O:x%Sb³⁺ can be seen with the increasing concentration of Sb³⁺. It has been considered that the increase of the Sb³⁺ concentration extends the distance between Sb³⁺ and Cl⁻ and decreases the magnitude of the crystal field, which brings about a smaller crystal field splitting and a blue shift of the Sb³⁺ emission spectra.^29,30 The full PL excitation spectrum of Rb₂ScCl₅·H₂O:Sb³⁺ (200–500 nm) monitored at 600 nm shows that there is no additional excitation peak lower than 300 nm (Fig. S2†), which corresponds to the 5p states of Sb³⁺ according to the DFT calculation. It is well documented that for Sb³⁺ with ns² excited electronic structure, the ground state is ¹S₀ and the excited state can be split into four energy states ¹P₁, ³P₀, ³P₁, and ³P₂. The excitation and emission of Sb³⁺ are usually attributed to the allowed transition between the ground state ¹S₀ and the excited state ³P₁.^31,32 However, the s–p transition of Sb³⁺ and STEs are exactly similar with a broad emission band and a large Stokes shift; thus, more evidence should be provided to elucidate the mechanism responsible for the luminescence of Sb³⁺ in Rb₂ScCl₅·H₂O and Rb₃ScCl₆.

Fig. 2 (a and b) PL excitation (λ_em = 600 nm) and emission spectra (λ_ex = 340 nm) of Rb₂ScCl₅·H₂O:x%Sb³⁺ with different x values. (c) Concentration-dependent emission wavelength and intensity of Rb₂ScCl₅·H₂O:xSb³⁺ (inset: PL decay curves of Rb₂ScCl₅·H₂O:2%Sb³⁺ and Rb₂ScCl₅·H₂O:20%Sb³⁺). (d) Linear fitting of the variation of PL intensity with excitation power density and (e) contour map of the temperature-dependent PL emission spectra of Rb₂ScCl₅·H₂O:Sb³⁺. (f) Emission wavelength and intensity of Rb₂ScCl₅·H₂O:Sb³⁺ as a function of measurement temperature in the range of 313–473 K.

The PL lifetimes of Rb₂ScCl₅·H₂O:Sb³⁺ with different Sb³⁺ concentrations have been measured (Fig. S3†) and all of them can be well-fitted with a singly exponential function. The PL lifetimes decrease monotonously from 1.06 to 0.34 ns with the Sb³⁺ concentrations increasing from 0.5 to 20 mol% (Table S3†), which is due to the increasing energy-transfer probability among the Sb³⁺ ions. The lifetimes of the Rb₂ScCl₅·H₂O crystals doped with 2 and 20 mol% Sb³⁺ fitted from the PL decay curves monitored at 600 nm are 0.48 and 0.34 ns, respectively (Inset of Fig. 2c). Such a short decay lifetime is more likely originated from the STE emission owing to the strong Jahn–Teller distortion-induced electron–hole recombination.³³

To rule out that the emission originated from permanent defects, we have investigated the function of the excitation power on the emission intensity at room temperature. The PL intensity of the emerged emission at 600 nm exhibits a linear dependence on the excitation power density up to 1000 mW cm⁻², suggesting that the emission is due to the intrinsic properties of Rb₂ScCl₅·H₂O:Sb³⁺ rather than permanent defects (Fig. 2d).^34,35 The effect of the high temperature on luminescence can be shown by the contour map and the point-line map (Fig. 2e and f). It can be seen that the PL intensities decrease as the measurement temperature increases from 313 to 473 K, which reflects the intense non-radiative recombination process in the high-temperature region. With the increase in the temperature, the emission band appears to slightly blue-shift from 600 to 585 nm, and this may be attributed to the enhanced electron–phonon coupling at the higher temperature.^8,13,19,24 After a heating–cooling cycle, the PL wavelength returns almost to the initial state and the intensity can reach 80% of the initial strength, indicating the excellent thermal stability of the Rb₂ScCl₅·H₂O:Sb³⁺ crystals (Fig. S4†). Moreover, the blue-shift has been found in the normalized excitation spectrum and the emission shrinks narrower at 77 K compared to that measured at 298 K (Fig. S5†), which is attributed to the reduction of thermally populated vibrational states at a low temperature.^36,37

Coordinative H₂O as a part of the Sc³⁺ octahedron is expected to have an important influence on the PL properties of the Rb₂ScCl₅·H₂O:Sb³⁺ crystals. To investigate the effect of coordinative H₂O, a series of Rb₂ScCl₅·H₂O:Sb³⁺ crystals have been sintered at different temperatures for 2 h in air. The digital photographs of the Rb₂ScCl₅·H₂O:Sb³⁺ crystals after heating treatment taken under UV light exhibit the obvious luminescence color changing from orange to white, and finally to cyan when the sintering temperature varies from 100 to 250 °C (Fig. 3a). The sintered crystals are insulated in transparent polyvinyl chloride bags to avoid the re-absorption of water from the atmosphere and characterized by the PL spectra and XRD promptly. The heating-treated Rb₂ScCl₅·H₂O:Sb³⁺ crystals show a sustained blue shift in the emission peak under 340 nm excitation with the treatment temperature increasing from 100 to 250 °C (Fig. 3b). The samples treated at 220 and 230 °C emit white light compensated by both orange and cyan emissions. The Commission International de L'Elcairage (CIE) coordinates of the samples are experienced from orange to cyan passing the white light region (Fig. S6†). The emission spectrum of the sample sintered at 230 °C can be separated into two bands centered at 590 and 605 nm from the Gaussian fitting PL spectrum (Fig. 3c). Additionally, only the cyan PL is observed in the sample treated at 250 °C.

Fig. 3 (a) Digital photographs of Rb₂ScCl₅·H₂O:Sb³⁺ after treatment at different temperatures taken under 365 nm UV light. (b) Normalized PL emission spectra (λ_ex = 340 nm) of Rb₂ScCl₅·H₂O:Sb³⁺ after treatment at different temperatures. (c) Gaussian fitting PL spectra of Rb₂ScCl₅·H₂O:Sb³⁺ excited by 340 nm after treatment at 230 °C.

To elucidate the fundamental reasons responsible for the changing of PL color, the phases of Rb₂ScCl₅·H₂O:Sb³⁺ crystals sintered at different temperatures have been characterized by XRD (Fig. 4a). The XRD patterns of the samples treated at a temperature lower than 200 °C can be well indexed to the pure phase of Rb₂ScCl₅·H₂O which is almost identical to that of Rb₂InCl₅·H₂O in previous work because the ionic radii of In³⁺ and Sc³⁺ are the same as 0.81 Å.³⁸ The Rb₂ScCl₅·H₂O phase is dominantly detected in the samples sintered at a temperature lower than 220 °C. An additional phase is present in those treated at a temperature higher than 230 °C. The dominant phase in the sample after heating Rb₂ScCl₅·H₂O:Sb³⁺ at 250 °C is supposed to be the Rb₃ScCl₆ phase which is similar to Rb₃InCl₆ (PDF#26-0936). The minor impurity denoted by asterisks is temporarily ascribed to the RbScCl₄ phase which is similar to RbInCl₄ (PDF#26-0935). Accordingly, the dehydration process at a high temperature is proposed as below:

Fig. 4 (a) XRD patterns of Rb₂ScCl₅·H₂O:Sb³⁺ treated at different temperatures. The asterisks denote the impurity of RbScCl₄. Structure projection of (b) Rb₃ScCl₆ and (c) Rb₂ScCl₅·H₂O crystals plotted using the software Diamond 3.1.

The regular octahedron [ScCl₆]³⁻ in the Rb₃ScCl₆ crystals is composed of central ions Sc³⁺ coordinated with six same Cl⁻ ions (Fig. 4b). When the Rb₃ScCl₆:Sb³⁺ crystals are exposed to moisture, coordinative H₂O replaces in one of the Cl⁻ ions and it is transformed into a distorted [ScCl₅(H₂O)]²⁻ octahedron. Consequently, cyan Rb₃ScCl₆:Sb³⁺ is transformed into orange Rb₂ScCl₅·H₂O:Sb³⁺ through the dehydration process at room temperature. In the structure of Rb₂ScCl₅·H₂O, the individual [ScCl₅(H₂O)]²⁻ octahedra are composed of central ions Sc³⁺ coordinated with five Cl⁻ ions and one O²⁻ from the H₂O molecule, which breaks the symmetry and subsequently produces a Jahn–Teller distortion (Fig. 4c). It is reasonably speculated that the cyan PL is emitted from Rb₃ScCl₆:Sb³⁺ based on the two facts: (i) the crystal Rb₃InCl₆:Sb³⁺ emits cyan PL under UV light, and the structure and XRD pattern of Rb₃ScCl₆ are fairly similar to those of Rb₃InCl₆; (ii) no PL is observed in Sb³⁺ doped RbInCl₄, and the structures of RbScCl₄ and RbInCl₄ are identical according to their XRD features.³⁸ Thus, it is reasonably supposed that the bright cyan PL originated from the Rb₃ScCl₆:Sb³⁺ crystals. The TG–DTA curve of Rb₂ScCl₅·H₂O:Sb³⁺ further demonstrates this transformation process (Fig. S7†). Nearly 7.8% weight loss has been observed up to 250 °C in the TG curve, which corresponds to the loss of coordinative H₂O in Rb₂ScCl₅·H₂O:Sb³⁺. The weight remains almost constant up to 700 °C, indicating the superior thermal stability of Rb₃InCl₆:Sb³⁺.

The PLE spectrum of the Rb₃ScCl₆:Sb³⁺ crystals displays a band at 340 nm with a small shoulder at ∼325 nm that has similar features to that of the Sb³⁺ doped perovskites (Fig. S8a†).^39,40 The cyan emission spectrum is centered at 500 nm with an FWHM of 100 nm and its decay curve can be fitted to a short-lived PL lifetime of 0.28 ns (98%) and a long-lived PL lifetime of 0.99 ns (2%) by a biexponential function, respectively (Fig. S8b†). Nevertheless, both lifetimes are in the nanosecond scope, which further infers that the recombination routes of Sb³⁺ in the two perovskites are ascribed to the STEs.³³

A comparison of the optical properties of Sb³⁺ in perovskite crystals formed as A₂BX₅·H₂O (A = Cs, Rb; B = In, Sc; and X = Cl, Br) and A₃BX₆ (A = Cs, Rb; B = In, Sc; X = Cl, Br) will give insights into optimizing the PL performance of Sb³⁺ (Table S4†). The undoped crystals A₂InX₅·H₂O (A = Rb, Cs; X = Cl, Br) exhibit weak PL with PLQYs lower than 33% and no PL has been observed in the primitive A₃InX₆ (A = Rb, Cs; X = Cl, Br) crystals. Upon doping Sb³⁺, the Cs₂InCl₅·H₂O:Sb³⁺ crystals exhibit orange PL with wavelengths located from 560 to 610 nm and PLQYs up to 95%. The PL of Cs₂InBr₅·H₂O:Sb³⁺ shifts to the deep red region (630–692 nm) compared to that of Cs₂InCl₅·H₂O:Sb³⁺ due to the expansion of crystal lattices resulted from the substitution of Br⁻ for Cl⁻. A similar redshift of PL from 745 to 823 nm has also been observed from the Sb³⁺ doped Cs₂ZnX₄ matrix with X varying from Cl⁻ to Br⁻.⁴¹ Besides halogens, the alkali cations in A-sites and the centers of the octahedron in B-sites have also significant influences on the PL spectra and PLQYs of the STEs from Sb³⁺. The A₃InX₆:Sb³⁺ (A = Rb, Cs; X = Cl, Br) crystals show cyan PL with wavelengths located from 497 to 522 nm, and the maximum PLQY of 95% is obtained in Rb₃InCl₆:Sb³⁺. Thus, it is concluded that the PL of Sb³⁺ can be switched by the transformation between A₂InX₅·H₂O and A₃InX₆ with the removal/addition of H₂O ligands.

The returning process from cyan to orange PL due to the transformation from Rb₃ScCl₆:Sb³⁺ to Rb₂ScCl₅·H₂O:Sb³⁺ can be realized by the hydration process under an ambient atmosphere. A series of digital photographs with a gradual change in the PL color are illustrated to depict the hydration process of Rb₃ScCl₆:Sb³⁺ stored under an ambient atmosphere and a closed atmosphere (Fig. 5a). After 60 min, the Rb₃ScCl₆:Sb³⁺ crystals exposed to air turn back to orange while those insulated from the air by a transparent glass lid maintain cyan, which definitely demonstrates the hydration process by absorbing H₂O from the air. Therefore, the above results indicate that the octahedral ligands (H₂O or Cl⁻) have a great influence on the position of the STE state and the recombination time of STEs.

Fig. 5 (a) Digital photographs of the cyan sample (top) exposed in and (bottom) isolated from air under an ambient atmosphere for 60 min. (b) PL emission spectra (λ_ex = 340 nm) of Rb₃ScCl₆:Sb³⁺ changing with the aging time. (c) The times consumed in the PL color of Sb³⁺ based on the transformation from Rb₃ScCl₆ to Rb₂ScCl₅·H₂O dependent on the contact areas (top) and relative humidity (bottom). (d) Waterfall plot of driven current-dependent EL spectra.

The gradual change in the PL emission color of Sb³⁺ in Sc-based perovskites has been observed from 600 to 500 nm after dehydration because of the gradual distortion of octahedra (Fig. 5b). Their Commission International de L'Elcairage (CIE) coordinates shift from cyan back to orange on account of the phase transformation from Rb₃ScCl₆ to Rb₂ScCl₅·H₂O (Fig. S9†). The rates of phase transformation are strongly dependent on the humidity of the environment and the contact areas of the samples exposed to the atmosphere. The times required for transformation are 1 h and 3 s by placing Rb₃ScCl₆:Sb³⁺ under an ambient atmosphere and in concentrated hydrochloric acid, respectively. The resulted products are confirmed further by XRD patterns, which indicate that the dominant phase Rb₂ScCl₅·H₂O is accompanied by the minor impurity of RbCl (Fig. S10†). Thus, the probable reaction equation for the reversible transformation is shown below:

To further discuss the rate of the transformation, the times taken in the transformation from Rb₃ScCl₆:Sb³⁺ to Rb₂ScCl₅·H₂O:Sb³⁺ under different contact areas and relative humidity are illustrated (Fig. 5c). As the relative humidity increases from 40% to 80%, the phase transition time decreases gradually from 102 to 29 min. Meanwhile, it shows an increasing time from 34 to 88 min with a decreasing contact area. Therefore, the larger the relative humidity and the bigger the contact area, the shorter the phase transition time required for the transformation from Rb₃ScCl₆:Sb³⁺ to Rb₂ScCl₅·H₂O:Sb³⁺. Briefly, this merit enables the two novel Sb³⁺ doped Sc-based perovskites to act as potential candidates for a sensitive luminescent sensor for probing humidity.³⁹

A typical WLED is fabricated with the orange phosphor Rb₂ScCl₅·H₂O:Sb³⁺ combined with the green phosphor BaSrSiO₄:Eu²⁺ (BSSO:Eu) and the blue phosphor BaMgAl₁₀O₁₇:Eu²⁺ (BAM:Eu) on a 360 nm InGaN chip. As shown in Fig. S11a,† the WLED device exhibits bright electroluminescence (EL) at an operating current of 40 mA, with a color coordinate of (0.3655, 0.3891) and a white-light correlated color temperature (CCT) of 4451 K (Fig. S11b†). Moreover, to confirm the excellent color stability, the EL spectra of the WLED driven under different operating currents are recorded as shown in Fig. 5d, and it is observed that they show similar shapes and positions except for the intensities.

To understand the Sb³⁺-induced optical properties and PL mechanism, the charge densities, band structure, and partial densities of states (DOS) of the primitive and Sb³⁺ doped Rb₃ScCl₆ and Rb₂ScCl₅·H₂O crystals are analyzed by density functional theory (DFT) calculations. The band gaps of the primitive Rb₃ScCl₆ and Rb₂ScCl₅·H₂O crystals are calculated to be 3.95 and 3.89 eV (Fig. S12 and S13†), respectively. After doping with Sb³⁺, the band gaps of the Sb³⁺-doped Rb₃ScCl₆ and Rb₂ScCl₅·H₂O crystals are reduced to 3.18 and 3.11 eV, (Fig. 6), respectively. The results indicate that the introduction of exotic Sb³⁺ ions into a low-dimensional crystal lattice of Rb₃ScCl₆ and Rb₂ScCl₅·H₂O would significantly decrease the bandgaps by the formation of Sb-s and Cl-p/O-p orbitals between the valence band (VB) and the conduction band (CB).^8,42 Additionally, the electronic states of the valence band maximum (VBM) of the Sb³⁺-doped Rb₃ScCl₆ crystals are comprised of the Sb-s and Cl-p orbitals and the VBM of Sb³⁺-doped Rb₂ScCl₅·H₂O comes from the Sb-s, Cl-p, and O-p orbitals from H₂O. No orbital hybridization between Sb 5s (or 5p) orbitals and Cl 3p (or O 2p) orbitals has been observed in the calculated results from DOS, which is quite different from that in Cs₂ZrCl₆:Sb³⁺ perovskite crystals.⁴³

Fig. 6 (a and d) The valence band maximum (VBM) and the conduction band minimum (CBM) associated charge densities of the (a) [SbCl₆]³⁻, [ScCl₆]³⁻ and (d) [SbCl₅(H₂O)]²⁻, [ScCl₅(H₂O)]²⁻centers in Rb₃ScCl₆:Sb³⁺ and Rb₂ScCl₅·H₂O:Sb³⁺ crystals, respectively. (b and e) Calculated electronic band structure and (c and f) density of states of (a–c) Rb₃ScCl₆:Sb³⁺ and (d–f) Rb₂ScCl₅·H₂O:Sb³⁺ crystals.

The conduction band minimum (CBM) states of both the Rb₃ScCl₆:Sb³⁺ and Rb₂ScCl₅·H₂O:Sb³⁺ crystals mainly come from the Sc-d orbitals rather than the Sb-p orbitals (Fig. 6, S14 and S15†). The charges in the CBM are concentrated at the center ions of Sc³⁺ and those in the VBM are distributed uniformly in the whole [SbCl₆]³⁻ octahedron, which signifies that the electrons occupied in the Sb-s orbitals are readily transferred to the Sc-d orbitals upon UV excitation. A higher energy is necessary to promote the Sb-s electrons up to the Sb-p orbitals and non-radiative transition to the Sc-d orbitals would occur subsequently. Thus, these results demonstrate that the dominant contribution for the PL of Sb³⁺-doped Sc-based perovskite crystals originates from the intrinsic excitons in the host lattice rather than in [SbCl₆]³⁻.⁴⁴ The electrons in the ground state are mainly distributed in the [SbCl₆]³⁻ octahedron, while the electrons in the excited state migrate to the [ScCl₆]³⁻ octahedron, which is accompanied by the strong electron–phonon coupling caused by the distortion of the octahedron units after doping.⁴⁵ The electrons in the Sb-s orbitals in the ground state can transfer to the Sc-d orbitals by creating trapping energy states upon the excitation of UV light and subsequently transfer back to the ground states. Through non-radiative relaxation and excited-state structural reorganization process, visible light with a large Stokes shift and broadband feature will be produced by the radiative transition process after the recombination of carriers.⁴³ As presented in the electronic band structure of Rb₂ScCl₅·H₂O:Sb³⁺ (Fig. 6e and f), the minimum energy for promoting the electrons from the VB to the CB is 3.11 eV (399 nm) and the Sb-p orbitals are located approximately at 4.06 eV (305 nm). There is no excitation band before 320 nm (Fig. S2†), which further indicates that the PL of Rb₂ScCl₅·H₂O:Sb³⁺ does not originate from the s–p transition of Sb³⁺ but from STEs induced by Jahn–Teller distortion.

Fig. 7 reveals the charge density distributions and local structures of the [SbCl₆]³⁻ octahedron in Rb₃ScCl₆:Sb³⁺ and the [SbCl₅(H₂O)]²⁻ octahedron in Rb₂ScCl₅·H₂O:Sb³⁺. The charge is uniformly distributed around the six Cl⁻ ions in [SbCl₆]³⁻ because of its symmetrical octahedral structure (Fig. 7a). The introduction of H₂O breaks the symmetrical structure of the octahedron and changes the charge density distribution in the octahedral structure by forming an O–Sb bond in a distorted octahedral structure of [SbCl₅(H₂O)]²⁻. Moreover, most of the charge is observed to be dragged toward H₂O in the [SbCl₅(H₂O)]²⁻ octahedron (Fig. 7b). The local octahedral structures of the [SbCl₆]³⁻ octahedron in Rb₃ScCl₆:Sb³⁺ indicate that [SbCl₆]³⁻ possesses the perfect structure, and the Sb–Cl bond length is about 2.65 Å, while the Cl–Sb–Cl angle is 87.6° (Fig. 7c). Meanwhile, the Sb–O bond length reduces to 2.439 Å, and the Cl–Sb–O angle reduces to 87.4° for [SbCl₅(H₂O)]²⁻ in Rb₂ScCl₅·H₂O:Sb³⁺, which demonstrates the further evident distortion of the lattice (Fig. 7d). The orbitals between the neighboring Cl-s and Sb-p show few overlaps and the charge carriers from electron transport can be negligible according to the calculated charge density of the best charge density isosurface plane of [SbCl₆]³⁻. Moreover, it is more difficult for the electrons in [SbCl₅(H₂O)]²⁻ to transfer because of the slighter overlap of orbitals from Cl-s, Sb-p, and O-p.

Fig. 7 (a and b) Charge density distribution and (c and d) local structures of (a and c) [SbCl₆]³⁻ in Rb₃ScCl₆:Sb³⁺ and (b and d) [SbCl₅(H₂O)]²⁻ in Rb₂ScCl₅·H₂O:Sb³⁺ crystals calculated by DFT.

Consequently, combining the theoretical calculation and the experimental characterization results of the optical properties, the luminescence mechanism of Rb₂ScCl₅·H₂O:Sb³⁺ and Rb₃ScCl₆:Sb³⁺ is illustrated in Fig. S16.† The electrons in the ground state are promoted to a higher excited state (ES1) and then trapped in [SbCl₆]³⁻ under excitation at 335 nm for Rb₃ScCl₆:Sb³⁺, which subsequently induced the STE1 emission of Rb₃ScCl₆:Sb³⁺ centered at 500 nm. In the same way, upon 340 nm excitation, the low energy will lead to the transfer of electrons to a lower excited state (ES2) and then trapped in [SbCl₅(H₂O)]²⁻ in Rb₂ScCl₅·H₂O:Sb³⁺. The electrons follow a self-trapped process to form STE2, and the recombination of electrons from the STE2 state to the ground state yields a wide orange emission at 600 nm with a large Stokes shift.

4 Conclusions

In summary, we have developed a new strategy to probe water content based on the orange/cyan emissive sensors of Sb³⁺ during the reversible phase transformation between the two novel rare-earth-based perovskite crystals Rb₂ScCl₅·H₂O and Rb₃ScCl₆ with high PLQYs and sensitivity. The in-depth analysis of the calculated energy of the bottom of the CBM suggests that the luminescence of Sb³⁺ in Sc-based perovskite crystals originates from STEs rather than an s–p transition of Sb³⁺ because the bottom of the CBM mainly consists of the 3d states of Sc³⁺ and the 5p states of Sb³⁺ which are far higher than the bottom of the CBM. After a hydration process through absorbing water in air, the [ScCl₆]³⁻ octahedron is converted into a distorted [ScCl₅(H₂O)]²⁻ octahedron accompanied by the bond length becoming shorter and the charge deviating preferentially toward the H₂O ligand, which causes the emission wavelength of Sb³⁺ to return from the cyan to orange region. This work provides a new water sensor by a switch of the PL color and sheds more light on the importance of the bandgap and charge distribution in determining the luminescence mechanism of Sb³⁺ in low dimensional perovskites.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research is financially supported by the National Natural Science Foundation (NSF) of China (52172152, 51932009, 52172166), the Key Projects of NSF of Zhejiang Province (LZ20E020003, LQ22B010003), the Wenzhou Major Scientific and Technological Innovation Project (ZG2020025), and the Applied Research Programs of Guangdong-Hong Kong-Macao Innovation Center sponsored by Guangzhou Development District.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2qi02221d

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