Introduction
Photocatalysts utilize light as the driving force in chemical reactions, and have great potential in both the production of solar fuels, e.g. H2 from water splitting, and organic transformations1. To make best use of the Sun’s energy output, a broad visible light absorption profile of the photocatalyst is desired. For heterogeneous semiconductor photocatalysts, the band gap of the semiconductor and potential sensitizers determines the light absorption properties of the catalyst system.
Herein, we present the functionalization of the layered semiconductor Sn3S7(trenH)2 (tren = tris[2-aminoethyl]amine) for potential photocatalytic applications. The compound consists of two-dimensional polymeric sheets of [Sn3S72-]n with molecular cations embedded in-between. The material is a violet light absorber with a band gap of approximately 3.0 eV. By partial exchange of the trenH+ ions for organic dyes, the light absorption properties of the large-band gap semiconductor are altered significantly2. The ion exchange is performed in solution, and a number of experimental parameters, e.g. temperature and solvent, have been studied. While solvents such as acetonitrile and acetone retain the crystal structure of Sn3S7(trenH)2, water causes an apparent structural decomposition of the framework. As many photocatalytic reactions are carried out in water, we have studied this amorphization in detail3, to determine whether the local structure and light absorption properties were maintained. X-ray total scattering and pair distribution function (PDF) analysis were used for in-depth characterization of the structural change.

Materials and Methods
Sn3S7(trenH)2 is synthesized by a solvothermal method. Time-resolved dye intercalation experiments were carried out by dispersing 0.2 g Sn3S7(trenH)2 in a 40 mL dye solution under stirring for 4 h. The dye concentration left in solution was determined by UV-Vis spectroscopy at different time intervals. Different organic dyes, i.e. Methylene Blue (MB) and Safranin T (ST) were tested, and the dye concentration, temperature, and solvent were varied. Optical properties of the dye-functionalized Sn3S7(trenH)2 were examined by Diffuse Reflectance Spectroscopy.
The time-dependent water-induced amorphization of Sn3S7(trenH)2 was studied by synchrotron X-ray total scattering. From the scattering data, PDFs were obtained, allowing for the study of the atomic-scale structure of powder samples, which had been dispersed in aqueous solution.

Results and Discussion
The intercalation of MB and ST in Sn3S7(trenH)2 was studied primarily in acetonitrile. At 60 oC and dye concentrations of 230 mg/L, the adsorption of MB is favored over that of ST by a factor of 17, despite a chemical resemblance of the two dyes. This is interpreted as a size-dependent diffusion of the dye molecules through the porous Sn3S7(trenH)2 framework. Temperature also plays an important factor in the dye uptake: Increasing the temperature of the solution from 20 oC to 60 oC increases the MB adsorption from 4.0 mg/g to 33.3 mg/g. The dye intercalation follows pseudo-second order kinetics. Diffuse reflectance spectra clearly demonstrate the increased visible light-absorption of dye-functionalized Sn3S7(trenH)2, cf. Figure 1.
Interestingly, the intercalation of MB in water is much more effective than it is in acetonitrile: At similar temperature and dye concentration, the dye uptake from water is almost 10 times greater than from acetonitrile. However, the crystalline Sn3S7(trenH)2 structure is not retained in water, as determined from powder X-ray diffraction patterns. Time-resolved PDFs shows that despite the loss of long-range order, the local order in the Sn3S7(trenH)2 structure does not change during the amorphization in water, cf. Figure 2. Diffraction data revealed a decrease of the interlayer distance between the [Sn3S72-]n sheets, which may be related to leaching of the trenH+ amines from the interior of the material, which results in a disordered stacking of the layers. Even so, the optical properties of the amorphisized Sn3S7(trenH)2 are not altered significantly, and its application as a photocatalyst may still be possible in aqueous solution.






Figure 1. Diffuse reflectance spectra of pristine and dye-functionalized Sn3S7(trenH)2. Figure 2. Pair Distribution Functions of Sn3S7(trenH)2 dispersed in H2O for different durations.


Significance
The present study showcases the possibility of soft chemical functionalization of Sn3S7(trenH)2 by ion exchange for visible light photocatalytic applications. The ion exchange can be performed under mild conditions and is useful for the introduction of molecular dye sensitizers or other functional species. We have shown that the structural transition from crystalline to amorphous, which occurs in water, is not associated with a complete decomposition of the [Sn3S72-]n sheets, but rather a disordering of the layers, and applications of Sn3S7(trenH)2 in aqueous solutions is therefore still possible.

References
1.Ismail, A. A.; Bahnemann, D. W., Sol. Energy Mater Sol. Cells, 2014, 128, 85-101.
2. Hvid, M. S.; Lamagni, P.; Lock, N., Sci. Rep., 2017, 7, 45822.
3. Hvid, M. S. et al., Manuscript in preparation.