Crystal structure
The structure of compound 1 was determined by single crystal X-ray studie. Compound 1 crystallized in the monoclinic space group P21/n (crystal data and refinement details in the Table 1). Compound 1 is a zero-dimensional cluster structure , which is composed of a transition metal complex [Ni(tren)(en)]2+, an isolated dimer anion [Sn2Se6]4-, and water molecules (Fig. 1). The [Ni(tren)(en)]2+ complex cations is formed by the octahedral coordination of Ni2+ ion chelated by four nitrogen atoms from a tren molecule and two nitrogen atoms from a en molecule (Fig. 2a). The octahedral configuration [Ni(tren)(en)]2+ is distorted, which can be seen from the N-Ni-N bond angle ranges from 81.23(14) to 176.66(14) ° and the Ni-N bond length ranges from 2.102(4) to 2.171(4) Å (see Table S1 for details of bond length and bond angle). These bond lengths and angles are close to the values reported in relevant literature, such as [Ni(dap)3]4[As10Cu2S18] [36], [Ni(en)(trien)]2Sn2Se6 [31], [Ni(dien)2]9Sb22S42·0.5H2O [37]. The two tetrahedra SnSe4 form a dimer [Sn2Se6]4- by common edge connection (Fig. 2b). The two tetrahedral SnSe4 in the dimer [Sn2Se6]4- are slightly deformed, as evidenced by the Se-Sn-Se angle, which ranges from 93.170(15) to 116.214(18)°. It can also be represented as a planar four-membered ring Sn2Se2, with each Sn atom having two terminal Sn-Se bonds. Sn-Se bridge bond length (2.5742(5) to 2.5775(5) Å) than at the end of Sn-Se bond length (2.4617(5) to 2.4823(5) Å) longer. This is similar to the Sn-Se bond containing [Sn2Se6]4- anion [31,33].
Powder X-ray diffraction and Thermal analyses
The powder X-ray diffraction patterns of compound 1 is shown in Fig. S1. The 2θ diffraction peaks obtained by the experiment are consistent with the simulation results of the single-crystal structures analysis, indicating the high purity of the compound 1, and sample can be used for further study. The thermal analyses of compound 1 was examined by TG-DTA in a N2 atmosphere from 40 to 800 ℃. As shown in Fig. 3, during the test, the TG curves revealed that compound 1 display multi-step weight losses. The first step of weight loss of 2.9% below about 210 ℃ (theoretical value of 2.8%), which is attributed to water molecules adsorbed in the samples, the second step of weight loss between 300 to 500 ℃ is corresponding to the removal of organic ligands with weight loss of 33.72% (theoretical value of 32.24%). With the increase of temperature, the weight loss continues until reaches equilbrium at 800 ℃.
Infrared spectra analysis
FT-IR spectra of compound 1 was recorded as solids in a KBr matrix. The sample was ground with dry KBr into a fine powder and pressed into translucent pellets. In the FT-IR spectra of compound 1 is shown in Fig. 4. For compound 1, the strong absorption peak can be attributed to the [Ni(en)(tren)]2+ organic cations and water molecules. We can see the ν(N–H) tensile vibration peaks at 3268, 3131 cm-1 and the δ(N–H) bending vibration strong absorption peak occur at 1650 cm−1; The ν(C–H) stretching vibration occur the relatively weak absorption peak at 2930 cm-1; The δ(C–H) bending vibration absorption peak appeared at 1458 cm−1; The ν(C–N), ν(C–C) stretching vibrations were observed the range of 1328~1022 cm−1 region and the δ(N–H), δ(C–H) bending vibration the weak absorption peaks in the range of 983~516 cm−1 region. The experimental results show that compound 1 has a tensile vibration peak at 3437 cm-1, thus confirming the presence of H2O molecule.
Optical properties
In the Fig. 5 shows the solid-state optical diffuse reflectance absorption spectra of compound 1, from which it can be seen that the optical band gap is 1.97 eV for compound 1, this value with other selenidostannate [Mn(dien)2]Sn3Se7·0.5H2O (1.89 eV), [Fe(tatda)]Sn3Se7 (1.93 eV)[32], [Fe(phen)3]Sn3Se7·1.25H2O (1.97 eV)[34] are similar. Compound 1 has semiconductor properties.
Photoelectric response property
The photoelectric response property were studied by using a standard three-electrode structure. The photocurrent density-time curve of Xe lamp (300 W) for multiple switching cycles is shown in Fig. 6. Repeatable photocurrent response was observed in compound 1, with xenon lamps periodically switched on and off under visible light, which indicate that the compound 1 is stable under visible light irradiation and have good reproducibility. We found an interesting phenomenon, the average photocurrent density of compound 1 about reaching 50 μA/cm2, significantly higher than that of other selenidostannates (Table 2), causes of this phenomenon may be associated band gap, the smaller band gap of compound 1 means that electrons need to absorb light with longer wavelength to complete the transition, thus reflecting better photoelectric response performance, these results indicate that transition-metal chalcogenides have broad application prospects in photocurrent field.
Table 2. Average photocurrent densities of other selenidostannates
Formula
|
Photocurrent
density (μA/cm2)
|
Ref.
|
[VII(dien)2]2[Sn2Se6]
|
5
|
[38]
|
[Zn(NH3)6]
[Ag4Zn4Sn3Se13]
|
0.06
|
[39]
|
[enH][Cu2AgSnS4]
|
0.05
|
[40]
|
(Cu2SnSe5)⋅
[Mn(H+-en)2(en)]
|
3.84
|
[41]
|
[Co(dien)2]2Sn2S6
|
15
|
[42]
|
[Ni(dien)2]2Sn2S6
|
10
|
[(Ni0.4Zn0.6)(en)6]
Sn2Se6
|
35
|
[(Mn1.67Zn0.33)(en)6]
Sn2Se6
|
20
|