Core Concepts
This computational study investigates the potential of Sr(1-x)CaxTaO2N solid solutions for photoelectrochemical water splitting by analyzing their band alignment with water redox levels, finding that most compositions are suitable for this application due to their favorable electronic structure.
Abstract
This research paper investigates the potential of Sr(1-x)CaxTaO2N solid solutions as photoelectrochemical water splitting materials using first-principles calculations based on density-functional theory (DFT).
Research Objective
The study aims to determine the band alignment of Sr(1-x)CaxTaO2N / H2O interfaces for different compositions (x = 0, 0.25, 0.5, 0.75, and 1) to assess their suitability for photoelectrochemical water splitting applications.
Methodology
The researchers employed two DFT approaches: the Full-Potential Augmented Plane Wave plus local orbital (FP-APW) method and the plane wave and pseudopotential method. They constructed supercells of Sr(1-x)CaxTaO2N with varying Ca/Sr ratios and simulated the interface with liquid water using classical molecular dynamics and DFT calculations. The band alignment was analyzed using a three-step method considering band bending at the interface.
Key Findings
- The study found that the I4/mcm structure is energetically more stable than the Pnma structure for the previously unreported composition of x = 0.5.
- The TB-mBJ method provided more accurate band gap predictions compared to the GGA PBE method, showing good agreement with experimental data.
- The band alignment analysis revealed that most Sr(1-x)CaxTaO2N compositions exhibit favorable band edge positions relative to the water redox levels, indicating their potential for photoelectrochemical water splitting.
- The study highlights the importance of considering band bending effects at the semiconductor/water interface for accurate band alignment predictions.
Main Conclusions
The researchers conclude that Sr(1-x)CaxTaO2N solid solutions, particularly those with x values other than 1, are promising candidates for photoelectrochemical water splitting applications due to their suitable band alignment. The study emphasizes the effectiveness of isovalent substitution at the A site of ABO2N oxynitrides for tuning band alignment and enhancing photoelectrochemical performance.
Significance
This research contributes to the field of photoelectrochemical water splitting by providing valuable insights into the electronic structure and band alignment of Sr(1-x)CaxTaO2N materials. The findings advance the understanding of these materials and their potential for clean energy applications.
Limitations and Future Research
The study acknowledges the limitations of computational modeling and suggests further experimental validation of the theoretical predictions. Future research could explore the impact of surface modifications, defects, and co-catalysts on the photoelectrochemical performance of Sr(1-x)CaxTaO2N materials.
Stats
The energy difference between the I4/mcm and Pnma structures for x = 0.5 is 0.118 eV.
The optimal number of layers for the orthorhombic CaTaO2N structure is n = 5.
The optimal number of layers for the tetragonal SrTaO2N structure is n = 3.
The water boxes were constructed with dimensions of 2a x 1b x 13.17 Å for Pnma and 2a x 2b x 13.17 Å for I4mcm.
The orthorhombic structures have a surface charge of ±8e.
The tetragonal structures have a surface charge of ±2e.
The calculated value for the H2O/H2 acceptor level relative to the HP in the water system is 0.7 eV.
The redox potential of water is 1.23 eV.
Quotes
"Computational quantum simulations have proven to be a suitable tool for understanding the microscopic processes that drive the physical properties of materials."
"This work shows that isovalent substitution in oxynitrides ABO2N between Sr and Ca at the A site to form solid solutions significantly contributes to tune and improve band alignment, and providing strong support to the methodology and modelling here proposed to study other oxynitride/H2O interfaces."