Aravind Kumar Chandiran

Designing a photocatalyst material with reduced recombination of photogenerated charges is one of the most important aspects of hydrogen generation through solar water splitting. Here, we report hydrogen generation using the TiO2/ultrathin g-C3N4 (U-g-CN) heterostructure fabricated using a unique in situ thermal exfoliation process. Multilayer g-CN is converted into U-g-CN having a high surface (∼190 m2/g) area by calcination at ∼550 °C through oxygen-induced exfoliation, which also forms a robust heterostructure with TiO2. In addition, the presence of g-CN also inhibits further growth of TiO2 nanoparticles, thereby retaining a high specific surface area. The presence of U-g-CN causes a redshift (∼0.13 eV) in the absorption edge of heterostructure compared to that of bare TiO2, which extends the light absorption capability. Addition of 40 wt. % of multilayer g-CN to TiO2 shows an enhanced H2 evolution rate, which is ∼15 times and ∼4 times higher compared to that of bare TiO2 and U-g-CN, respectively. Photoluminescence (PL) and time-resolved PL (TRPL) studies indicate a reduced recombination rate of photogenerated charge carriers with an increase in the average lifetime from 10.53 (TiO2) to 13.32 ns (TiO2/U-g-CN40). The interfacial charge transport characteristics studied through impedance spectroscopy reveal a reduced charge transfer resistance at the semiconductor-electrolyte interface, which facilitates faster charge separation due to the heterostructure formation. The band edge positions are estimated through flatband potential from the Mott-Schottky measurements and optical absorption data, indicating a type-II heterojunction. More light absorption and enhanced separation of photogenerated charges at the heterojunction interface lead to better photocatalytic H2 generation.

Halide perovskite materials with ABX3 show excellent photovoltaic properties, but their stability remains a serious concern. On the other hand, the double perovskite with two formula unit A2BB′X6 shows excellent stability, but their visible light absorption properties are inferior, which restricts them from being employed in solar energy conversion devices. The incorporation of dopant ions in the crystal structure is found to be an effective strategy to introduce the visible light absorption to these materials. In this work, copper is systematically substituted in the place of Ag+ (B site) in three different halide double perovskites, Cs2Ag(Bi/In/Sb)Cl6, and we explored their structural, optical, and electrochemical properties in detail. The role of copper in tuning the properties is found to be different in each of these materials. Only a small fraction of the dopant could be introduced in Cs2AgInCl6 (CAIC) and Cs2AgSbCl6 (CASC), and no copper could be incorporated in Cs2AgBiCl6 (CABC). The presence of Cu+ in the CAIC and CASC crystals (i) distorted the octahedra formed by [InCl6]3- and [SbCl6]3- and (ii) introduced states above the valence band affecting the optical properties. The Cu 3d orbitals are introduced very close to the valence band of CAIC, leading to a change in the absorption onset from 400 to 580 nm. However, for CASC the copper orbitals are pushed toward the conduction band, resulting in near IR absorption with an onset at ∼1300 nm. The color of CAIC changed from white to yellow, and CASC changed from yellow to black upon substitution of copper. Combining structural, optical, and electrochemical characterization experiments, we explained the role of copper in improving the visible light absorption in halide double perovskites.

Structural distortion in halide perovskites is important to tune the optical properties of the materials. The octahedra formed by metal cations and halide anions in these classes of materials remain symmetric; however, the introduction of asymmetry provides enormous opportunities to improve the photoluminescence emission and excited-state lifetimes for their application in white light emitters. In this work, we have systematically introduced asymmetry in vacancy-ordered halide triple perovskite materials Cs3M2X9 (M = Bi3+, Sb3+; X = Cl−, Br−, I−) by mixing trivalent sites in three different halide compounds. The Raman and FT-far-IR measurements were used to investigate the distortion introduced in these materials. The distortion is shown to (i) enhance self-trapped excitonic emission, which is broad and intense leading to emission in the complete visible region and (ii) improve excited-state lifetimes. This strategy to create distortion and its proven ability to improve light emission will find application in light-emitting diodes.

Halide perovskite materials recently attracted wide attention for light-emitting applications. The intense white light emission and excited state lifetimes greater than 1 μs are the hallmarks of a good light-emitting material. Here, we provide a clear design strategy to achieve both of these aforementioned properties in a single material via the introduction of octahedral asymmetry in halide double perovskites Cs2AgMCl6 through iso-trivalent substitution at the M site. In the substituted Cs2AgMCl6, the presence of mixed M3+ sites distorts the [AgCl6]5- octahedra, affecting the parity of the valence and conduction band edges and thereby altering the optical transitions. The distortion also creates a local polarization that leads to an effective photogenerated carrier separation. Considering perovskite series with three M3+ cations, namely Bi3+, In3+ and Sb3+, the mixed trivalent cationic compounds with specific ratios of In3+ and Bi3+ show white light emission with intensity nearly 150 times larger than that of the parent compounds, and are characterised by excited state lifetimes nearing 1 μs. Using single crystal X-ray diffraction, far-infrared absorption, steady-state and time-resolved photoluminescence, bias-dependent photoluminescence, P-E loop traces and density-functional theory calculations, we hence demonstrate the role of octahedral distortion in enhancing white light emission and excited state lifetimes of halide double perovskites.

In this work, stable Cs2AgMCl6(M = Bi,In,Sb) double perovskite materials are successfully employed as photoanodes for solar water oxidation. These materials show an extraordinary oxidative stability, where Cs2AgInCl6 and Cs2AgBiCl6 are stable between 0 and 1.2 V (vs Ag-AgCl) and Cs2AgSbCl6 between 0 and 0.75 V (vs Ag-AgCl) over 100 cycles of electrochemical cycling. This enables us to employ these materials for photoelectrochemical (PEC) water oxidation in CH3CN and H2O, with and without IrOx cocatalyst. All three materials show PEC activity, with Cs2AgInCl6 showing the highest performance. At 1.23 V (vs a reversible hydrogen electrode), a photocurrent density of 0.5 mA cm-2 is observed, and with an applied overpotential of 600 mV, the photocurrent increases to about 0.75 mA cm-2. From the band gap of Cs2AgInCl6, the estimated theoretical maximum current is around 0.82 mA cm-2. So, Cs2AgInCl6-coated IrOx gives nearly 60% and about 90% of the theoretical maximum photocurrent at zero bias and at an applied overpotential, respectively. The device is shown to be stable and reproducibility is confirmed.