Hydrothermal growth and characterization of large Rb₂SnBr₆ double perovskite crystals: a promising semiconductor material for photocatalysis and optoelectronics

Abstract

In this study, we present the growth of large (millimeter- and centimeter-scale) crystals of Rb₂SnBr₆ double perovskite via a hydrothermal process. The crystals and powders were successfully synthesized, yielding light-yellow products, and subjected to comprehensive characterization using powder and single crystal X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS) point analysis, and UV-Vis diffuse reflectance spectroscopy. Previously, methods such as solution growth, evaporation, and gel techniques have been employed to synthesize Rb₂SnBr₆. However, none of these approaches have successfully yielded large crystals on the millimeter- or centimeter-scale. Our experimental results reveal that Rb₂SnBr₆ is a semiconductor with a bandgap of 2.97 eV. This wide bandgap not only suggests high stability and low defect levels but also positions Rb₂SnBr₆ as a highly promising candidate for advanced applications in photocatalysis, photovoltaics, and optoelectronics. The ability to grow large-sized crystals with such favorable electronic properties highlights the material's potential for integration into scalable technologies, paving the way for further research and development in energy conversion and optoelectronic devices.

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Introduction

Double halide perovskites, represented by the general formula A₂BX₆ (A = alkali-earth metals, B = transition metals, and X = halide), have emerged as a fascinating class of materials due to their diverse structural properties and extensive potential applications.^1,2 These materials have garnered significant attention for their roles in photocatalysis,³ optoelectronics,⁴ photovoltaics,⁵ thermoelectric,⁶ and nonlinear optical devices⁷ owing to their unique electronic, optical, and magnetic characteristics. These materials exhibit a unique crystal structure that contributes to their high absorption coefficients, long carrier diffusion lengths, and tunable bandgaps, making them ideal candidates for various applications, notably in photovoltaic cells and light-emitting devices.⁸ Even though stability is the main issue with halide perovskite, many are very stable at ambient temperature. Stability is a primary concern, as these materials can degrade under environmental stressors like moisture, heat, and ultraviolet light.^9,10 Additionally, the presence of lead in most high-performing perovskites poses environmental and health risks, necessitating the development of lead-free alternatives.^11,12

Current research focuses on improving the stability of halide perovskites through compositional engineering, surface passivation, and encapsulation techniques.¹³ Moreover, significant efforts are being made to discover and develop non-toxic, lead-free perovskites that do not compromise performance.¹⁴ Lead-free halide perovskites are emerging as a promising alternative to traditional lead-based perovskites, addressing environmental and health concerns of lead toxicity. These materials seek to retain the desirable properties of lead halide perovskites, such as high absorption coefficients, long carrier lifetimes, and tunable bandgaps, while eliminating the harmful effects of lead. Lead-free halide perovskites typically replace lead with other metals such as tin, germanium, bismuth, and antimony.^14–18 Among these, tin-based perovskites are the most studied due to their structural similarity to lead perovskites and their promising optoelectronic properties.^19,20 Despite their potential, Sn-based perovskite optoelectronic devices still underperform compared to their lead counterparts.²¹ Sn perovskites differ from their Pb counterparts in that they are prone to oxidation, attributed to their different 5s²p² configuration and, thus, the lack of lanthanide contraction.¹³ Therefore, Sn(II) is converted to Sn(IV) more readily when it is exposed to oxidation sources such as oxygen or solvents like dimethyl sulfoxide (DMSO).²² As a result, synthesizing stable single tin halide perovskites, like ASnX₃, remains highly challenging. This has led to vacancy-ordered double perovskites, such as A₂SnX₆, emerging as promising alternatives.^23,24

Among the tin-based A₂SnX₆ (A = K, Rb, Cs; and X = Cl, Br, I) families, there are a few experimental reports on Rb₂SnBr₆. The first experimental report, by Costeanu et al., came out in 1927²⁵ after that, Ketelaar et al.²⁶ described the crystal structure of Rb₂SnBr₆ using XRD analysis. The third report was published in 1980 by Suib et al.²⁷ synthesize Rb₂SnBr₆ using a gel process. Numerous theoretical studies of Rb₂SnB₆ have been published since.^6,28–32 In 2022, Ganesan et al.³³ reported a synthesis of nanocrystals of Rb₂SnBr₆ double perovskite nanocrystals by the evaporation process. Furthermore, their targeted materials were synthesized RbSnBr₃ nanocrystals, which spontaneously oxidized to stable Rb₂SnBr₆ nanocrystals. Finally, in 2023, Glockzin et al., directly synthesized Rb₂SnBr₆ powder by an evaporation process.³⁴ The synthesis of Rb₂SnBr₆ double perovskite might have been underreported for several reasons, one of which is the poor stability of Sn-based perovskites. Tin can readily oxidize from Sn²⁺ to Sn⁴⁺, leading to the degradation of the resulting materials. This instability can make it challenging to synthesize and study these materials in a reproducible manner.²² The synthesis of high-quality Rb₂SnBr₆ can be more challenging compared to other perovskites. Factors like the precise control of stoichiometry, temperature, and atmosphere during synthesis can be more demanding, leading to fewer successful reports. Researchers may prioritize other lead-free perovskite materials that offer better stability and more accessible synthesis, such as bismuth or antimony-based perovskites. These materials might be more attractive due to their potentially better performance and stability.^35,36

We successfully synthesized the Rb₂SnBr₆ millimeter and centimeter-sized large crystals using a one-step hydrothermal process. We were initially motivated to grow Rb₄Sn₃Br₂I₈ from the work of Gong et al.,³⁷ who synthesized Rb₄Sn₃Cl₂Br₈ for the optoelectronic application. However, our analysis revealed that instead of forming the Rb₄Sn₃(Br₂I₈) phase, it formed Rb₂Sn(Br_5.5I_0.5), which indicates that Rb₂SnBr₆-type perovskite is the more favorable product in this quaternary system. We later synthesized the Rb₂SnBr₆ phase using a stoichiometric amount of precursors. Going forward, we will use (Rb : Sn : X) ratio notation to indicate the nominal stoichiometry used where relevant: Rb₂SnBr₆ (2 : 1 : 6) or Rb₂SnBr₆ (4 : 3 : 10).

Experimental procedure

Chemicals

Doped and pristine Rb₂SnBr₆, Rb₂SnCl₆, and Rb₂SnI₆ were synthesized using a one-step hydrothermal synthesis technique. The chemicals used for the synthesis are: RbBr (99%, Strem), RbCl (99.8%, Sigma-Aldrich), RbI (99.9%, Sigma-Aldrich), SnBr₄ (99%, Sigma-Aldrich), SnBr₂ (99.2%, ThermoFisher), SnCl₄ (98%, Sigma-Aldrich), SnI₄ (99%, ThermoFisher), HBr (48%, Sigma-Aldrich), HCl (37%, VWR) and HI (57%, Sigma-Aldrich). All chemicals utilized in this process are employed without further purification.

Hydrothermal synthesis

Two compositions were selected for the study: Rb₂SnBr₆ (2 : 1 : 6) and Rb₄Sn₃Br₁₀ (4 : 3 : 10). Stoichiometric amounts of RbBr and SnBr₄ were measured into a 25 mL Teflon liner within a glove box due to SnBr₄'s hygroscopic nature. Subsequently, 2.0 mL of an HBr solution (HBr : H₂O = 1 : 3) was added to the mixtures. The role of HBr is to maintain the acidic condition of the system. The Teflon liner was then sealed within a stainless-steel autoclave and placed in a furnace. The temperature was maintained at 220 °C for 24 hours, followed by a gradual reduction of 3 °C per hour to promote the growth of finer crystals. Light yellow crystals of millimeter size were extracted from the Rb₄Sn₃Br₁₀ (2 : 1 : 6) composition, while yellow crystals of centimeter size were obtained from Rb₂SnBr₆ (4 : 3 : 10). The reaction for the formation of Rb₂SnX₆ is given below:

Powder XRD

PXRD characterization was performed using a Rigaku Miniflex 600 diffractometer employing Cu-Kα radiation (λ = 1.5434 Å). The sample holders were comprised of zero-background silicon plates. The finely ground sample powders were placed on round sample holders and pressed with a glass slide to secure the samples. Diffraction measurements were taken while rotating the sample holder at 15 rpm. The Rietveld refinement was done using the open-source Python package Py-GASA-II, developed by Brian Toby from Argonne National Laboratory.³⁸

Single crystal XRD

SCXRD data were collected on a Rigaku XtaLAB Synergy-S Dual Source diffractometer equipped with a PhotonJet Mo-microfocus source (λ = 0.71073 Å) and a HyPix-6000HE detector. Data reduction was performed with CrysAlisPro 65; subsequent data processing was also performed in CrysAlisPro. Using the SCALE3 ABSPACK scaling algorithm 66, empirical absorption corrections were applied to the data. Empirical and numerical (Gaussian) absorption corrections, determined by face-indexing and integration, were applied to the data. The structure was solved by applying the intrinsic phasing in SHELXT and refined by full-matrix least-squares techniques against F² (SHELXL) in the Olex2 graphical user interface.³⁹

Diffuse-reflectance

The bandgaps were calculated from UV-Vis absorption spectra using Tauc plots. UV-Vis measurements of the finely ground powder samples were performed with a Cary 5000 UV-Vis-NIR instrument, using KBr as a standard reference sample.

EDS

The energy-dispersive X-ray spectroscopy (EDS) point analysis was done by a Hitachi S-4800 high resolution scanning electron microscopy. The voltage, current and magnification were used 15 kV, 20 mA and 5000× for EDS measurement. Generally, we chose one spot for the EDS point; however, 3 spots were also chosen for the pristine Rb₂SnBr₆.

Thermogravimetric analysis

Thermogravimetric analysis (TGA) was performed using the Water™ Discovery TGA 5500 instrument. TGA was performed on the single crystals of Rb₂SnBr₆ (2 : 1 : 6) and Rb₂SnBr₆ (4 : 3 : 10) under a nitrogen atmosphere in a Platinum pan. The temperature range for the TGA measurement was set from 0 °C to 1000 °C.

Results and discussion

Rb₂SnBr₆ exhibits a face-centered cubic structure with the space group Fm3̄m (space group #225), which corresponds to the K₂PtCl₆ structure type. Typically, halide double perovskite compounds with the formula A₂BX₆ exhibit a vacancy-ordered structure characterized by isolated [BX₆] octahedra that form a 12-fold coordination environment with discrete X anions. In this arrangement, A atoms are coordinated by 12 halide atoms, and B atoms are coordinated by 6 halide atoms, resulting in a face-centered cubic crystal structure (Fig. 1).

Fig. 1 Crystal structure of Rb₂SnB₆ (Fm3̄m), showing the polyhedral filling with the 12-sided [RbBr₁₂] prism and [SnB₆] tetrahedra.

The synthesis of air- and moisture-stable Rb₂SnBr₆ presents significant challenges, as evidenced by the scarcity of published experimental data. To date, only four experimental studies on Rb₂SnBr₆ halide double perovskite have been reported.^25–27,33 Although several theoretical studies on Rb₂SnBr₆ predict various properties such as photocatalysis, thermoelectricity, optoelectrical behavior, and photoluminescence,^4–6 the limited experimental data have impeded the validation of these properties. Here we present the first to report the successful growth of large crystals of Rb₂SnBr₆ double perovskite, ranging from millimeter to centimeter scales.

The successful synthesis of large crystals of Rb₂SnBr₆ double perovskite was achieved via a hydrothermal process. Details for the synthesized crystals can be seen in Table 1. Traditionally, tin(II) bromide (SnBr₂) is employed in the synthesis of Rb₂SnBr₆; however, the oxidative nature of Sn leads to the oxidation of Sn(II) to Sn(IV) in ambient conditions.²² To mitigate this issue, SnBr₄ was used instead of SnBr₂, yielding remarkable results. We successfully synthesized several batches of air- and moisture-stable large crystals of Rb₂SnBr₆, ranging from millimeters to centimeters in size (Fig. 2). The as-synthesized Rb₂SnBr₆ is light yellow; however, Cl and I-doping on the Br site change colors from yellow to dark yellow and dark orange, respectively (Fig. 2). A few inclusions can be observed with naked eye with a length of 0.5–4 mm (approximately 5 to 10% of the total crystal volume (Fig. S1†)); however, we could not determine with full certainly if they are single or poly-crystalline domains (Fig. S1†). Using optical microscopy faceting could not be detected in the domains. There was no evidence of this domain formation in 10% Cl-doped Rb₂SnBr_5.4Cl_0.6 crystal, suggesting that 10% Cl-doping might promote defect-free crystal growth. In order to grow such large single crystals of better quality it would be important to precisely control the synthesis and cooling temperatures, changing the solvent concentration, and introducing a seed crystal growing technique.

Fig. 2 Images of as-synthesized centi- and millimeter-sized single crystals of pristine and doped Rb₂SnBr₆: (a) Rb₂SnBr₆ (4 : 3 : 10), (b) Rb₂SnBr₆ (2 : 1 : 6), (c) Rb₂SnBr_5.4Cl_0.6 (2 : 1 : 6) (d) Rb₂SnBr_4.5Cl_1.5 (2 : 1 : 6), (e) Rb₂SnBr₃Cl₃ (2 : 1 : 6) (f) Rb₂SnBr_1.5Cl_4.5 (2 : 1 : 6) (g) Rb₂SnBr_5.5I_0.5 (4 : 3 : 10)(h) Rb₂SnCl₆ (2 : 1 : 6).

Table 1 Crystallographic data for the halide perovskite crystals

Phase	Rb2SnCl6 (2 : 1 : 6)	Rb2SnBr6 (2 : 1 : 6)
CSD-number	2381630	2381629
Space group	Fm3̄m
λ (Å)	Mo-Kα: 0.71073
T (K)	105(2)	101(2)
a (Å)	10.0195(1)	10.5767(6)
V (Å^−3)	1005.86(3)	1183.2(2)
Z	2
ρ (g cm^−3)	3.317	4.317
Absorption correction	Multi-scan
μ (mm^−1)	13.671	30.531
θ (°)	3.49 < θ < 53.74	3.34 < θ < 46.11
Data/param.	364/6	121/6
R1	1.62	3.01
wR2	3.73	6.86
Goodness-of-fit	1.137	1.247
Diff. peak/hole (e Å^−3)	1.592/−1.491	1.558/−1.908

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The synthesis of Rb₂SnBr₆ crystals requires careful control of growth and nucleation to produce high-quality materials with desirable properties. Key factors include adjusting precursor concentrations, temperature, solvent conditions, and growth kinetics. High supersaturation encourages nucleation, leading to multiple tiny crystals, while lower supersaturation promotes the growth of larger crystals. Maintaining an optimal precursor ratio (RbBr : SnBr₄) is crucial, as excessive tin precursor concentration can induce nucleation. Temperature control is essential, with lower temperatures suppressing nucleation and higher but controlled temperatures promoting growth. Solvents like DMF or ethylene glycol ensure controlled supersaturation, while crystallographic factors influence the development of defined facets. Post-synthesis treatments such as annealing can further refine crystal quality. By balancing these factors, we can produce Rb₂SnBr₆ crystals with controlled sizes and minimal defects.

Initially, our research aimed at synthesizing Rb₄Sn₃Br₂I₈ perovskite materials; however, Rb₂SnBr₆ emerged as the more favorable phase for this synthesis technique. We synthesized a series of Rb₂SnX₆ (X = Cl, Br, and I) double perovskites and characterized them using single crystal X-ray diffraction (SXRD) and powder crystal X-ray diffraction (PXRD). The PXRD results matched the reference data set ICSD-158956, as shown in Fig. 3. In addition, we doped Rb₂SnBr₆ (2 : 1 : 6) with Cl and I and characterized by Rietveld refinement, PXRD (Fig. S2 and Table S2†), and EDS point analysis (Fig. S3 and S4†). Our effort to synthesize the double perovskite Rb₄Sn₃Br₂I₈ ultimately produced Rb₂SnBr_5.5I_0.5, which was confirmed by SCXRD as well as a peak shift to lower angles in the PXRD (Fig. S5†) data. The Rietveld refinement of the powder crystal XRD also confirms the successful formation of the Rb₂SnBr₆ (Fig. 3).

Fig. 3 Analyzed powder XRD data: (a) Rietveld refinement plot of the Rb₂SnBr₆ (4 : 3 : 10) double perovskite halide. Rietveld refinement data: Fm3̄m, a = 10.678 Å, V = 1217.71 ų, R_wp = 4.38%, and (b) powder crystal XRD of Rb₂SnBr₆ (2 : 1 : 6) and Rb₂SnBr₆ (4 : 3 : 10), which are a close match to each other.

In addition to validating previous experimental techniques, we attempted to synthesize Rb₂SnBr₆ perovskite using an evaporation process previously reported by Ganesan et al.³³ at 120 °C. We utilized both SnBr₂ and SnBr₄ as the sources of tin. Our experimental results indicate that evaporation can only produce micron-sized powders (Fig. S6 and S7†). The powder XRD analysis revealed the formation of RbSnBr₆ phases mixed with the corresponding raw materials phase of SnBr₂ and SnBr₄. Moreover, our hydrothermal study for the fabrication of large Rb₂SnBr₆ crystals produces a similar result at 150 °C. The formation of the halide double perovskite was confirmed by comparing the PXRD data with reference standards (ICSD#158956). The synthesized Rb₂SnBr₆ appeared light yellow when SnBr₄ was used, but it turned dark orange when SnBr₂ was the tin source. We tried to synthesize Rb₂SnBr₆ perovskite at 120 °C by the hydrothermal process; however, the reaction was relatively unsuccessful, with the majority of products being unreacted reactants (Fig. S8†). It can be concluded that 120 °C is not enough for the reaction to reach the optimum pressure to facilitate the reaction and grow the crystals by hydrothermal process. We compare and arrange several synthesis techniques previously applied to fabricate Rb₂SnBr₆ crystals in Table S1.†

Meanwhile, we were successfully able to synthesize colorless crystals of Rb₂SnCl₆ double perovskite, however, our efforts to grow stable Rb₂SnI₆ crystals were unsuccessful. Although we managed to produce yellow, needle-like crystals of Rb₂SnI₆, they degraded to black crystals within 24 hours when exposed to air. Despite numerous experimental reports of nano- and micron-sized Rb₂SnCl₆, our large sample demonstrated excellent stability in air and moisture. SCXRD and PXRD analyses confirmed the successful formation of the cubic phase of Rb₂SnCl₆ double perovskite (Table 1 and 2 and Fig. S9†).

Table 2 Several synthesis techniques for the growing of Rb₂SnBr₆ halide perovskite single crystal

Single crystal growing process	Gel growth	Evaporation	Solution	Hydrothermal
Ref.	27	33	26	Current work
Precursor materials	RbBr (aq.), SnBr3/SnBr4, CH3COOH	RbBr, SnBr2	RbBr, SnBr2	RbBr, SnBr4, HBr
Solvent	H2O	H2O/DMF	H2O	H2O
Temperature (°C)	25–65	120	150	220
Crystal size	Millimeter	Nano	Unknown	Milli to centimeter
Phase	Cubic (Fm3̄m)	Cubic (Fm3̄m)	Cubic (Fm3̄m)	Cubic (Fm3̄m)
Crystal color	Yellow white	White	Yellow	Light yellow
Yield percentage	Unknown	Unknown	Unknown	70–80%

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Through numerous experimental iterations, we successfully synthesized large crystals in each successful attempt. However, some of these crystals exhibited twining properties. To confirm the formation of the target phase, we performed elemental analysis using energy-dispersive X-ray spectroscopy, which showed a close match with the nominal composition Rb_2.2(2)Sn_1.04(4)Br_5.51(44) (Fig. S11†).

We calculated the optical bandgap of the synthesized compounds, as shown in Fig. S12.† The UV-Vis absorption data were used to calculate the bandgap using the Tauc equation:

(αhυ)^1/n = A(hυ − E_g) (2)

where α is the absorption coefficient, h is Planck's constant, ν is the frequency, A is a proportional constant, and E_g is the optical bandgap energy.

The direct and indirect bandgaps of the pristine and doped samples are displayed in Fig. S12.† The calculated data indicate that both pristine and doped Rb₂SnBr₆ halide double perovskites possess bandgaps of close to 3 eV, making them promising candidates for applications in photocatalysis, optoelectronics, thermoelectric, and photoluminescence (PL). We present both direct and indirect bandgaps in Fig. S12† but we do not claim these perovskites to be direct-band gap semiconductors without further investigation. The pristine Rb₂SnBr₆, which is light yellow in color, has a bandgap of 2.97 eV, consistent with the literature. The direct bandgap of I-doped Rb₂SnBr_5.5I_0.5 (2 : 1 : 6) is reduced to 2.29 eV, corresponding to its color spectrum. The bandgap of Cl-doped Rb₂SnBr₃Cl₃ (2 : 1 : 6) increased to 3.14 eV due to the presence of Cl even though the intrinsic Rb₂SnCl₆ has a direct bandgap of 4.42 eV. Replacing Br with Cl decreases the ionic radius. Consequently, lattice contraction takes place. Replacing halogen introduces the nephelauxetic effect, which reduces the effective nuclear charge of the host system. It could be one of the major points in reducing the bandgap of 50% Cl-doped Rb₂SnBr₃Cl₃ (2 : 1 : 6).

In addition, we performed the thermogravimetric analysis (TGA) of both samples Rb₂SnBr₆ (2 : 1 : 6) and Rb₂SnBr₆ (4 : 3 : 10) (Fig. S13†) from 0 °C to 1000 °C. It reveals that both the centi- and millimeter-sized Rb₂SnBr₆ crystals are stable to around 300 °C, and then begin to decompose. Rb₂SnBr₆ (2 : 1 : 6) started to decompose at 303.45 °C while the decomposition of Rb₂SnBr₆ (4 : 3 : 10) started at 376 °C. A phase transition seems to be occurred for Rb₂SnBr₆ (2 : 1 : 6) at 304 °C. At close to 500 °C, both of them lose more than 50% of their total weight. After increasing the temperature, the decomposition continues, and at the end of 1000 °C, 13.46% of Rb₂SnBr₆ (2 : 1 : 6) and 4.5% of Rb₂SnBr₆ (4 : 3 : 10) remain.

Conclusion

Our experimental results and subsequent data analysis demonstrate the successful fabrication of large milli- and centimeter-sized Rb₂SnBr₆ halide double perovskite via the hydrothermal process. Powder X-ray diffraction (PXRD) and single-crystal X-ray diffraction (SXRD) analyses confirmed the formation of the cubic structure with the space group Fm3̄m (number 225). PXRD and energy-dispersive X-ray spectroscopy (EDS) elemental analysis verified the successful formation of Rb₂SnBr₆. All Cl- and I-doped Rb₂SnBr₆ halide double perovskites demonstrated stability in air and moisture. The optical bandgap, calculated from absorption data, indicates wideband semiconductor properties, suggesting potential applications in photocatalysis, electrocatalysis, optoelectronics, and photoluminescence. Due to time constraints, we were unable to test the physical properties and applications of these crystals. Our future work will focus on investigating the photocatalytic and electrocatalytic activities of Rb₂SnBr₆ halide double perovskite.

Author contributions

The manuscript was written through the contributions of all authors.

Data availability

Additional experimental details of the hydrothermal synthesis, PXRD, EDS, UV-Vis diffuse-reflectance, and TGA data. Further details of the crystal structure investigations may be obtained from the joint CCDC/FIZ Karlsruhe by quoting the CSD deposition numbers: 2381629 and 2381630.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by a Rutgers University, Newark startup grant (R. H., H. Z. A., R. J., G. A.). The X-ray diffractometer was purchased with support from the National Science Foundation Grant No. [2018753] (R. A. L.).

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Footnote

† Electronic supplementary information (ESI) available: Additional experimental details of the hydrothermal synthesis, powder XRD, EDS, and TGA data. CCDC 2381629 and 2381630. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4dt02712d

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