Cs2SnCl6: To Emit or to Catalyze? Te4+ Ion Calls the Shots

Abstract A low concentration of Te4+ doping is found to be capable of endowing the lead‐free Cs2SnCl6 perovskites with excellent photoluminescence quantum yield (PLQY), while further increasing Te4+ concentration leads to PLQY deterioration. The mechanism behind the improved PLQY is intensively studied and reported elsewhere. However, little work is conducted to understand the decreased PLQY at high doping levels and to explore its implications for non‐PL‐related applications. Here, it is demonstrated that the Te4+‐incorporated Cs2SnCl6 can be promising candidate for efficient CO2 photocatalysis. An optimum photocatalytic performance is achieved when Te4+ concentration reaches as high as 50%, at which point significant PL quenching has occurred. Through a detailed spectral characterization, such concentration‐dependent functionality is attributed to systematic changes in both electronic and local crystal structure, which allow a robust regulation of excitation energy relaxation channels. These findings expand the scope of available photocatalysts for CO2 reduction and also inform synthetic planning for the preparation of multifunctional Pb‐free metal halide perovskites.


Synthesis of Cs 2 Sn 1-x Te x Cl 6 (0 ≤ x ≤ 1) by hydrochloric acid-assisted precipitation method (HAAPM)
For Cs 2 Sn 1-x Te x Cl 6 (x = 0, 0.05, 0.25, 0.5, 0.75, 1), x mmol of TeCl 4 and 1-x mmol of SnCl 4 were dissolved in 5 mL HCl for A solution, and 2 mmol of CsCl is dissolved in 2 mL HCl for B solution.Then the B solution is rapidly injected into A solution with vigorously stirring, and the Cs 2 Sn 1-x Te x Cl 6 is precipitated at the bottom.Finally, the obtained powder (undoped sample: white color; doped sample: yellowish color) was filtered out and washed with ethanol before drying at 60 ℃ overnight.

Synthesis of Cs 2 Sn 1-x Te
x Cl 6 (x = 0.05 and 0.5) by hydrothermal method (HTM) Preparation method for solution A, B is same as described in section 1.2.The two solutions were mixed together and then transferred to a polytetrafluoroethylene lined reactor.After reaction at 180 °C for 12 h, the reactor was allowed to be slowly cooled to room temperature.
The obtained powder was filtered out and washed with ethanol before drying at 60 ℃ overnight.

Characterization
Powder X-ray diffraction (XRD) measurement was performed on a PANalytical Empyrean diffractometer equipped with Cu Kα X-ray (λ =1.54056 Å) tubes, and the acquisition was done for every 0.05° increment.Inductively coupled plasma optical emission spectrometer (ICP-OES) was performed on PerkinElmer ICP-OES 7300DV.The scanning electron microscopy (SEM) measurements were performed by using the Quanta 250 FEG.Steady-state absorption spectra were recorded using a UV-vis (SHIMADZU UV2600) spectrometer.
Optical diffuse reflectance measurement was performed by equipping with an integrating sphere at room temperature and BaSO 4 as the 100% reflectance reference.The reflectance data were converted to absorption according to the Kubelka-Munk equation.The chemical states of the samples were determined by X-ray photoelectron spectroscopy (XPS) on ESRCALAB250Xi, Thermo Fisher Scientific.The binding energies are referenced to the C 1s peak at the binding energy of 284.8 eV.PLQY were measured by using an integrating sphere on an Absolute PL Quantum Yield Spectrometer (C9920-02G, Japan).Steady-state PL and time-resolved PL (TRPL) measurements were performed on a FLS1000 spectrofluorimeter (Edinburgh Instruments Ltd, UK) with 380 nm excitation wavelength.The TRPL spectra were recorded using time-correlated single photon counting (TCSPC) technology and fitted with di-exponential function (for x = 0.05, 0.25 and 0.5) and single-exponential function (for x = 0.75 and 1).The average PL lifetimes (τ ave ) were calculated as follows: The electrochemical impedance spectroscopy (EIS) was carried out using a threeelectrode cell (CHI 660D, Shanghai Chenhua) with a Pt foil counter electrode and a saturated Ag/AgCl reference electrode at the open circuit potential using a frequency ranged from 10 6 Hz to 10 -1 Hz.The working electrode is prepared by dip-coating method.About 10 mg of the photocatalyst is dispersed in 1.5 mL of acetonitrile and 10 μL Nafion solution to form slurry.
Then, 4 μL of the slurry was dip-coated on the glassy carbon electrode.Acetonitrile solution with 0.1 mol/L of tetrabutylammonium hexafluorophosphate (TBAPF 6 ) was used as the electrolyte.

Photocatalytic CO2 reduction
A solid-gas mode was adopted to evaluate the photocatalytic CO 2 reduction activity of Cs 2 Sn 1-x Te x Cl 6 (x = 0, 0.05, 0.25, 0.5, 0.75, 1).The measurements were performed within a 50 mL sealed Pyrex bottle filled with CO 2 and H 2 O vapor.Specifically, the vacuum treated clean sample films (catalyst mass of about 10 mg) and 30 μL water were put into the bottle which

Figure S6 .
Figure S6.Characterization of the PLQYs of Cs 2 Sn 1-x Te x Cl 6 , including their excitation spectra of reference and emission spectra.

Figure S9 .
Figure S9.Mass spectra of reaction products of photocatalytic CO 2 reduction using Cs 2 Sn 0.5 Te 0.5 Cl 6 as catalyst.The peaks at m/z = 36 can be assigned to 18 O 2 .

Figure S15 .
Figure S15.The trend of the average particle sizes of Cs 2 Sn 1-x Te x Cl 6 with increasing Te 4+ ion concentration.

Figure S17 .
Figure S17.Comparison of high-resolution XPS spectra of (a) Sn 3d and (b) Cl 2p of Cs 2 Sn 0.5 Te 0.5 Cl 6 , tested in dark conditions and under in-situ light irradiation, respectively.

Figure S18 .
Figure S18.Characterization of the PLQYs of Cs 2 Sn 0.95 Te 0.05 Cl 6 synthesized by HAAPM and HTM.

Table S5 .
The PLQY of Cs 2 Sn 0.95 Te 0.05 Cl 6 synthesized by HTM were tested five times.