Hydrothermal Synthesis and Crystal Structure of a Novel Bismuth Oxide: (K0.2Sr0.8)(Na0.01Ca0.25Bi0.74)O3.

A novel distorted perovskite-type (K0.2Sr0.8)(Na0.01Ca0.25Bi0.74)O3 was prepared by a hydrothermal method using the starting materials NaBiO3·nH2O, Sr(OH)2·8H2O, Ca(OH)2, and KOH. Single-crystal X-ray diffraction of the novel compound revealed a GdFeO3-related structure belonging to the monoclinic system of the space group Cc with the following cell parameters: a = 11.8927 (17) Å, b = 11.8962 (15) Å, c = 8.4002 (10) Å, and β = 90.116 (9)°. The final R-factors were obtained as R 1 = 0.0354 and wR 2 = 0.0880 (using all the data). K+ and Sr2+ ions were distributed at four types of A-sites. On the other hand, four Bi5+-sites (Bi1, Bi2, Bi3, and Bi4) were occupied by four Ca2+ ions (Ca1, Ca2, Ca3, and Ca4), and the first three B-sites were occupied predominantly by Bi5+ with Na+ ions. The forth B-site was occupied predominantly by the Ca2+ ion with Bi5+ ions. Two types of B-sites, thus forming tilted distorted (Na/Ca/Bi)O6 and (Bi/Ca)O6 octahedra, have an ordering of 3:1 represented as (K/Sr)4(Na/Ca/Bi)3(Bi/Ca)O12. The distorted (Na/Ca/Bi)O6 and (Ca/Bi)O6 octahedra formed a perovskite-type network by corner sharing with features closely matching those of a GdFeO3-type structure. The novel compound is the first example of a perovskite-type bismuth oxide containing only Bi5+ in a system without a Ba atom and has a unique ordering (3:1) of the B site. The compound showed photocatalytic activity for phenol degradation under visible light irradiation.

Low-temperature hydrothermal reactions have become an effective method for fabricating novel inorganic materials.[1−10] Given the availability of numerous chemical species and varying reaction conditions, a wide variety of compounds have been synthesized by a hydrothermal method with NaBiO3·nH2O as a starting reagent.[11−26] The most prominent feature of the hydrothermal reaction is that only Bi5+ is produced, as evidenced by the preparation of previously reported compounds such as ABiO3 (A = Li, Ag),[11,12] (Ba0.54K0.46)4Bi4O12,[27] ABi2O6 (A = Zn, Mg, Cd, Sr, Ba),[13−16] A2Bi2O7 (A = Ca, Sr),[17] and (Sr0.75Bi0.25)2Bi2O6.83.[28] Obtaining this result via a high-temperature solid–state reaction process is unprecedented. As Bi5+ is not stable at high temperature, it is not easy to generate a Bi5+ oxide through a high-temperature solid-state reaction method, except the compounds containing alkaline or barium atoms with Bi3+ in their crystal structure.[29,30] A high-pressure and high-temperature technique (2 GPa, 700 °C) was used to prepare superconducting Sr0.4K0.6BiO3 with an average Bi valence of +4.6 and a superconducting transition temperature Tc of ∼12 K.[31] Several bismuthates containing Bi5+ exhibited significant photocatalytic activity under visible light irradiation.[32−34] Moreover, the enhanced photocatalytic performance under visible light was also observed for Bi-based semiconducting heterostructured compounds AgBr/Bi2Sn2O7[35] and (Sr0.6Bi0.305)2Bi2O7/TiO2.[36] By exploring novel bismuthates, we were the first to synthesize a novel bismuth oxide (K0.2Sr0.8)(Na0.01Ca0.25Bi0.74)O3 via a low-temperature hydrothermal reaction method using NaBiO3·nH2O. The structural information, thermal behavior, and photocatalytic activity of this novel bismuth oxide are detailed in this article.

The X-ray diffraction (XRD) pattern showed a single phase for the newly synthesized bismuth oxide (Figure S1). The SEM image shown in Figure showed a cubic shape with an approximate size of 20 μm for this hydrothermally synthesized compound. Elemental mapping revealed an almost homogeneous distribution in the microscopic range for the constituent elements (Figure S2).

Figure 1

SEM micrograph of hydrothermally synthesized (K0.2Sr0.8)(Na0.01Ca0.25Bi0.74)O3.

The crystal structure of the novel compound was successfully determined by single-crystal XRD, where the final R-factors were obtained as R1 = 0.0354 and wR2 = 0.0880 (using all the data). The structural information, structural parameters, and interatomic distances are detailed in Tables S1, S2, and S3, respectively. The newly synthesized bismuth oxide has a GdFeO3-type structure. The crystal structures of (K0.2Sr0.8)(Na0.01Ca0.25Bi0.74)O3 and GdFeO3 are presented in Figure a,b, respectively. The unit cell parameters of (K0.2Sr0.8)(Na0.01Ca0.25Bi0.74)O3 are related to ao (for an ideal cubic perovskite) as a = 2√2ao, b = 2√2ao, and c = 2ao; and the corresponding relationships for GdFeO3 are a = √2ao, b = √2ao, and c = 2ao. The GdFeO3 crystal structure has been detailed elsewhere.[37] The supercell of this compound is formed by a unique ordering (3:1) of the B site as described later. The B-sites in both compounds clearly form an array through corner-shared distorted octahedra; extending along the c-axis. For both compounds, octahedral BiO6 and FeO6 are surrounded by A-site cations (Sr2+/K+ and Gd3+, respectively), which are encompassed by four octahedra.

Figure 2

Crystal structure of (a) hydrothermally prepared (K0.2Sr0.8)(Na0.01Ca0.25Bi0.74)O3 and (b) GdFeO3 compounds along the c-axis.

There are four B-sites in the crystal structure, namely, Bi1, Bi2, Bi3, and Bi4, among which Bi1, Bi2, and Bi3 sites are occupied with Ca and Na atoms, that is, Na1/Ca1/Bi1, Na2/Ca2/Bi2, and Na3/Ca3/Bi3, respectively, as marked by the violet ball presented in Figure S3a. These B-sites are occupied predominantly by Bi5+ ions. Besides, Ca and Bi atoms are distributed over the crystallographic site Bi4, as indicated by the blue ball in Figure S3a. This B-site is occupied predominantly by Ca2+ ions. Two types of B-sites, thus forming tilted distorted (Na/Ca/Bi)O6 and (Bi/Ca)O6 octahedra, have an ordering of 3:1 represented as (K/Sr)4(Na/Ca/Bi)3(Bi/Ca)O12. It is well known that the double perovskite-type structure has a 1:1 ordering of the B site as seen in Ba2MBiO6 (M: rare-earth metals).[38] A high-pressure phase of La4Cu3MoO12 was reported to have a 2:2 ordering of the B site represented as La4Cu2(Cu/Mo)2O12.[39] To the best of our knowledge, (K0.2Sr0.8)(Na0.01Ca0.25Bi0.74)O3 is the first example for the 3:1 ordering in the B site.

The mean Na/Ca/Bi–O distances for Na1/Ca1/Bi1–O, Na2/Ca2/Bi2–O, and Na3/Ca3/Bi3–O of 2.12, 2.10, and 2.09 Å, respectively, are consistent with the mean Bi–O distances (2.08–2.13 Å) of some previously reported pentavalent bismuth oxides.[11−17,40] Interestingly, the mean bond length of Ca4/Bi4–O of 2.25 Å is higher than the Bi–O length in pentavalent bismuthates because the large Ca mostly occupies this site with Bi. However, the mean bond length of Ca4/Bi4–O is somewhat smaller than the Ca–O bond length in Ca2Sb2O7[41] and Ca2Bi2O7.[17] Therefore, the valence of Bi is clearly 5, which is in good agreement with the value of 4.99 calculated from the chemical formula of this novel bismuth oxide: (K0.21+Sr0.82+)(Na0.011+Ca0.252+Bi0.745+)O32–. This valence is also confirmed by the X-ray photoelectron spectroscopy (XPS) analysis presented in Figure . Two peaks were fitted at 159.2 and 164.5 eV for Bi 4f7/2 and Bi 4f5/2, respectively, and assigned to Bi5+, which is almost similar to the results reported for NaBiO3.[42] By contrast, the K+ and Sr2+ ions are distributed over the four distinct Sr-sites (Sr1, Sr2, Sr3, and Sr4), that is, K1/Sr1, K2/Sr2, K3/Sr3, and K4/Sr4, as indicated by the green ball in Figure S3a. Table S3 lists the mean K/Sr–O distances for K1/Sr1–O, K2/Sr2–O, K3/Sr3–O, and K4/Sr4–O as 2.81, 2.78, 2.67, and 2.76, respectively, which are analogous to the Sr–O bond lengths of the previously reported compounds.[17,43,44]

Figure 3

Comparison of the X-ray photoelectron spectrum of the Bi 4f5/2 and Bi 4f7/2 signals for hydrothermally synthesized (K0.2Sr0.8)(Na0.01Ca0.25Bi0.74)O3 and NaBiO3.

Figure is a graphical representation of the unit cell volume versus the sum of the ionic radii of A and B cations for GdFeO3-type distorted perovskites[35,43,45−48] and perovskite bismuth oxides.[9,10,22,23,27,31,38,49−53] Our newly synthesized (K0.2Sr0.8)(Na0.01Ca0.25Bi0.74)O3 exhibits a linear relationship with GdFeO3-type compounds, again confirming the structural similarities with this group. The results for the novel compound also lie within the region for superconducting bismuth oxides; therefore, the novel compound belongs to this group but may not be a superconductor because only Bi5+ is found at the B-site. Shannon’s ionic radius[54] is used for the A and B atoms.

Figure 4

Unit cell volume vs the sum of ionic radii of A and B atoms for GdFeO3-type distorted perovskites and perovskite bismuth oxides.

A thermogravimetric (TG) analysis was carried out to determine the thermal behavior of the hydrothermally synthesized sample. The TG curve (Figure S4) displayed a total mass loss of 2.62% up to 700 °C. There was no discernible loss of mass below 500 °C, suggesting no H2O molecules or OH groups were associated with our novel product. The sample started to decompose above 500 °C because of O2 release, indicating the reduction of Bi5+ to Bi3+. The XRD patterns of the samples heated to room temperature (RT), 400, and 700 °C are presented in Figure S5. No change was observed upon heating the sample to 400 °C, but the sample decomposed at 700 °C, justifying the TG analysis. A completely different XRD pattern was obtained for the decomposed sample at 700 °C, and a novel phase was found similar to previously reported monoclinic (space group C2/m) Bi4Ca1.87Sr1.63O9.5[55] and Bi2Sr0.68Ca1.07O4.75.[56]

The optical absorption spectrum of (K0.2Sr0.8)(Na0.01Ca0.25Bi0.74)O3 is presented in the inset of Figure S6. The absorption edges lie within the visible region. Using a Tauc plot depending on (hαν)2 and hν with the assumption of direct transitions, band-gap energy can be determined.[34] Using the Tauc plot to calculate the band-gap energy for polycrystalline samples has been reported to yield appropriate values for monazite-type oxides.[57] The band gap energy was found to be <1.4 eV, as shown in Figure S6.

The photocatalytic activity was measured by the decomposition of phenol at an initial concentration of 20 ppm, using 0.15 g of the prepared sample in 50 mL of ultrapure water. Figure shows the time-dependent C/C0 under dark conditions and visible light irradiation (λ ≥ 420 nm). The suspensions were magnetically stirred in the dark (60 min) to observe phenol adsorption. The phenol concentration changed negligibly during this period. The photocatalytic activity for phenol degradation under visible light irradiation exhibited by the sample (green line) was slightly lower than that of commercial NaBiO3 (red line). The blank phenol solution remained almost unchanged under both dark and light conditions (purple line), confirming the photocatalytic activity of our prepared sample.

Figure 5

Time-dependent photocatalytic degradation of phenol for (K0.2Sr0.8)(Na0.01Ca0.25Bi0.74)O3.

Figure S7 shows the temperature-dependent magnetic susceptibility (4πM/H) of hydrothermally synthesized (K0.2Sr0.8)(Na0.01Ca0.25Bi0.74)O3. The figure clearly shows that the compound exhibited temperature-independent diamagnetism over the 3–300 K temperature range. The electrical resistivity at room temperature was on the MΩ order and increased with decreasing temperature. This behavior suggested that the synthesized sample was semiconducting.

In summary, a novel bismuth oxide (K0.2Sr0.8)(Na0.01Ca0.25Bi0.74)O3 was produced by a hydrothermal route at 240 °C using NaBiO3·nH2O as one of the starting reagents. Single-crystal XRD was used to determine the crystal structure of this compound, revealing the first example for 3:1 ordering in the B site for the perovskite-type structure. The morphology and compositional homogeneity of the novel compound were determined by SEM and EDX. The novel compound exhibited structural stability up to 500 °C, despite the low-temperature synthesis. This bismuth oxide exhibited photocatalytic activity for phenol degradation under visible light irradiation.

Experimental Details

The single crystal of (K0.2Sr0.8)(Na0.01Ca0.25Bi0.74)O3 was synthesized by employing a facile hydrothermal route in aqueous solutions, taking NaBiO3·nH2O (1 g), Sr(OH)2·8H2O, and Ca(OH)2 (molar ratio = 1:1:1) as the starting materials with 15 g of KOH and just 1 mL of distilled water. All the starting chemicals were purchased from Kanto Chemical Co., Inc. (Tokyo, Japan). The starting reagents in proper quantity according to their molar ratio were kept in a Teflon-lined autoclave (70 mL) and compactly closed for heating at 240 °C in an oven for 7 days to carry out the reactions. After finishing the reaction process, the autoclave was cooled and depressurized in a water bath. Thereafter, the solid sample was separated by filtration, frequently washed with distilled H2O, and dried 6–8 h at 60 °C. A size-selective gravity separation technique was used to separate the single crystals and remove the impurities by filtering. In the hydrothermal synthesis process, water is used as the reaction medium through which the raw chemicals come into contact and start the reaction process upon heating. Herein, Na atoms of the starting material NaBiO3·nH2O were fully replaced by K and Sr atoms at the A-site, whereas Bi atoms were partially substituted by Ca and Na atoms. Because Na has a smaller atomic radius than K and Sr but close to Ca, it occupied the B-site with Ca, despite the fact that Na has a very low occupancy. Hence, the final compound was formed.

The primary phase of the sample was explored by XRD on a Rigaku X-ray diffractometer (Miniflex 600) with nickel-filtered Cu Kα radiation (λ = 1.54056 Å). A single-crystal X-ray diffractometer (Bruker D8 QEST) with monochromatic Mo Kα radiation (λ = 0.71073 Å) was used to investigate the crystal structure with refinements. The morphology and elemental distributions were observed with a scanning electron microscope (SEM; Hitachi TM3030Plus). The crystal structure was visualized by using the widely used software VESTA.[58] The oxidation state of bismuth was identified by XPS (JEOL-9 200). The thermal stability of the prepared sample was tested by using TG analysis (Rigaku ThermoPlus) with a heating rate of 10 °C/min from room temperature up to 700 °C. The absorption of this sample was measured by UV–vis spectroscopy (V-550, JASCO). The aqueous solution (20 ppm) of phenol was prepared with ultrapure water, and the catalyst was added at a concentration of 3 g/L. The solution was magnetically stirred in the dark and under visible light irradiation from a 300 W Xe lamp (UXR-300DU, Ushio Inc.) with a 420 nm sharp cut filter (GG420, SHIBUYA OPTICAL Co., Ltd.). The time-dependent phenol concentration was evaluated by liquid chromatography (JASCO LC-2000). The temperature-dependent DC magnetic susceptibility was measured using PPMS (Quantum Design).