Acid‐ and Base‐Stable Cs₂Pt(Cl,Br)₆ Vacancy‐Ordered Double Perovskites and Their Core–Shell Heterostructures for Solar Water Oxidation

Abstract

The stability of the absorber materials in an aqueous medium is the key to developing successful photoelectrochemical (PEC) solar fuel devices. The halide perovskite materials provide an opportunity to tune desired optoelectronic properties and show very high photovoltaic power conversion efficiency. However, their stability is poor as they decompose instantly in an aqueous electrolyte medium. Here the most stable vacancy ordered double perovskites Cs2PtCl6 and Cs2PtBr6, which remain intact in a wide range of pH values between 1 and 13 is reported. These materials also possess excellent absorption properties covering a significant portion of the visible spectrum. Like conventional ABX3 materials, these ultrastable materials offer tunability in optical properties via mixed halide sites. Through anion exchange, the conversion of Cs2PtCl6 to Cs2PtBr6 through core–shell conversion mechanism is shown. The latter led to the formation of type‐II heterostructures. The electrochemical properties of these materials are investigated in detail and their ability to carry out solar water oxidation on an unprotected photoanode, with photocurrent density of >0.2 mA cm−2 at 1.23 V (vs. RHE) is demonstrated.

Highly Stable and Panchromatic Light Absorbing Cs2OsX6 (X = Cl−, Br−, I−) Vacancy Ordered Perovskites as Photoanodes for Solar Water Oxidation

“…[ 31 ] Similar shift in the spectra was reported for Cs 2 PtX 6 . [ 15 ] These materials are found to be in the order of a few microns as evident from the SEM micrographs shown in the inset of Figure 1d.…”

“…It has to be noted that the diffraction patterns of Cs 2 OsX 6 also appear similar to XRD patterns of Cs 2 PtX 6 and Cs 2 SnX 6 . [15,26] A typical octahedron possesses six different vibrational modes out of which two are IR active due to asymmetric bending and stretching. [30] The FTIR vibrational properties of Cs 2 OsX 6 (X = Cl − , Br − , I − ) were studied with the help of FTfar-IR spectroscopy, and plots are shown in Figure 1d.…”

“…[ 20 ] In one of our recent works, the anion exchange method was proposed to create core‐shell heterostructures with core and shell possessing different halogens. [ 15 ] These structures were shown to enhance photogenerated carrier separation. Similarly, the Sn/Te ratio in Cs 2 Sn 1−x Te x I 6 has been shown to affect conductivity, carrier mobility, and carrier concentration.…”

Stable Cs2ReX6 (X – Cl, Br) vacancy-ordered perovskites for solar water splitting

“…Similar to previous reports on Pt, Sn or Pd based compounds, based on the similarity in the PXRD patterns, the present materials can be assigned to the Fm 3m space group. 34,38,39 The structure contains an isolated octahedron based on (ReX 6 ) 2− with Cs + occupying the void spaces. Ideally, this crystal structure is similar to the ABX 3 structure where two B sites with divalent charges are replaced by one tetravalent rhenium and a vacancy (Fig.…”

“…32,33 Specically the Cs 2 PtX 6 family of compounds has been employed for PEC water oxidation where the materials possess panchromatic absorption covering over 700 nm (for bromide and iodide halides). 34 Similar to platinum containing materials, the palladium containing compounds are also shown to be stable in aqueous media and demonstrated photocatalytic water splitting. 35 In this work we report two rhenium compounds in the family of vacancy ordered perovskites, Cs 2 ReX 6 (X -Cl, Br) that remain extremely stable under ambient and harsh pH conditions.…”