Zinc Titanium Nitride Semiconductor toward Durable Photoelectrochemical Applications

First-Principles Studies of the Electronic and Optical Properties of Zinc Titanium Nitride: The Role of Cation Disorder

Cation disorder is an established feature of heterovalent ternary nitrides, a promising class of semiconductor materials. A recently synthesized wurtzite-family ternary nitride, ZnTiN2, shows potential for durable photoelectrochemical applications with a measured optical absorption onset of 2 eV, which is 1.4 eV lower than previously predicted, a large difference attributed to cation disorder. Here, we use first-principles calculations based on density functional theory to establish the role of cation disorder in the electronic and optical properties of ZnTiN2. We compute antisite defect arrangement formation energies for one hundred 128-atom supercells and analyze their trends and their effect on electronic structures, rationalizing experimental results. We demonstrate that charge imbalance created by antisite defects in Ti and N local environments, respectively, broadens the conduction and valence bands near the band edges, reducing the band gap relative to the cation-ordered limit, a general mechanism relevant to other multivalent ternary nitrides. Charge-imbalanced antisite defect arrangements that lead to N-centered tetrahedral motifs fully coordinated by Zn are the most energetically costly and introduce localized in-gap states; cation arrangements that better preserve local charge balance have smaller formation energies and have less impact on the electronic structure. Our work provides insights into the nature of cation disorder in the newly synthesized semiconductor ZnTiN2, with implications for its performance in energy applications, and provides a baseline for the future study of controlling cation order in ZnTiN2 and other ternary nitrides.

Tolerance Factor and Phase Stability of the KCoO2-Type AMN2 Nitrides

In order to help understand the structural stability of KCoO2-type ternary nitrides AMN2, referring to perovskite structure, a tolerance factor t is proposed to describe the size effect on the phase/symmetry options of the experimentally accessible AMN2 nitrides. This leads to a range of t values above 0.946 for structurally stable KCoO2-type AMN2 nitrides with t values around 0.970 for the orthorhombic and tetragonal phase boundary. In contrast, most of AMN2 nitrides exhibit α-NaFeO2-type structure with t ∼ 0.898-0.946 and cations ordered or disordered rocksalt structure while t below 0.898. Employing the proposed criterion, the structure formation for other ternary AMN2 compositions with lanthanum and alkaline earth cations for the A sites were predicted, which was testified through the synthesis attempts and complemented by formation energy evaluations. The efforts to synthesize the ternary Lanthanide and alkaline earth-based AMN2 nitrides were unsuccessful, which could associate the structural instability with the large formation energies of lanthanide nitrides LaMN2 and the greater tolerance factor of 1.048 for BaTiN2. The experimentally already synthesized AMN2 nitrides could be categorized into three types with different tolerance factors, and scarce AMN2 nitrides with lower formation energies would be accessible using different synthetic routes beyond the traditional solid-state synthesis method.

Mechanistically Guided Materials Chemistry: Synthesis of Ternary Nitrides, CaZrN2 and CaHfN2

Recent computational studies have predicted many new ternary nitrides, revealing synthetic opportunities in this underexplored phase space. However, synthesizing new ternary nitrides is difficult, in part because intermediate and product phases often have high cohesive energies that inhibit diffusion. Here, we report the synthesis of two new phases, calcium zirconium nitride (CaZrN2) and calcium hafnium nitride (CaHfN2), by solid state metathesis reactions between Ca3N2 and MCl4 (M = Zr, Hf). Although the reaction nominally proceeds to the target phases in a 1:1 ratio of the precursors via Ca3N2 + MCl4 → CaMN2 + 2 CaCl2, reactions prepared this way result in Ca-poor materials (CaxM2-xN2, x < 1). A small excess of Ca3N2 (ca. 20 mol %) is needed to yield stoichiometric CaMN2, as confirmed by high-resolution synchrotron powder X-ray diffraction. In situ synchrotron X-ray diffraction studies reveal that nominally stoichiometric reactions produce Zr3+ intermediates early in the reaction pathway, and the excess Ca3N2 is needed to reoxidize Zr3+ intermediates back to the Zr4+ oxidation state of CaZrN2. Analysis of computationally derived chemical potential diagrams rationalizes this synthetic approach and its contrast from the synthesis of MgZrN2. These findings additionally highlight the utility of in situ diffraction studies and computational thermochemistry to provide mechanistic guidance for synthesis.

Electronic Properties of Zn2V(1-x)NbxN3 Alloys to Model Novel Materials for Light-Emitting Diodes

We propose the Zn2V(1-x)NbxN3 alloy as a new promising material for optoelectronic applications, in particular for light-emitting diodes (LEDs). We perform accurate electronic-structure calculations of the alloy for several concentrations x using density-functional theory with meta-GGA exchange-correlation functional TB09. The band gap is found to vary between 2.2 and 2.9 eV with varying V/Nb concentration. This range is suitable for developing bright LEDs with tunable band gap as potential replacements for the more expensive Ga(1-x)In(x)N systems. Effects of configurational disorder are taken into account by explicitly considering all possible distributions of the metal ions within the metal sublattice for the chosen supercells. We have evaluated the band gap's nonlinear behavior (bowing) with variation of V/Nb concentration for two possible scenarios: (i) only the structure with the lowest total energy is present at each concentration and (ii) the structure with minimum band gap is present at each concentration, which corresponds to experimental conditions when also metastable structures are presents. We found that the bowing is about twice larger in the latter case. However, in both cases, the bowing parameter is found to be lower than 1 eV, which is about twice smaller than that in the widely used Ga(1-x)In(x)N alloy. Furthermore, we found that both crystal volume changes due to alloying and local effects (atomic relaxation and the V-N/Nb-N bonding difference) have important contributions to the band gap bowing in Zn2V(1-x)NbxN3.

Type-I heterojunction destruction by In situ formation of Bi2S3 for split-type photoelectrochemical aptasensor

Development of new strategies in photoelectrochemical (PEC) sensors is an important way to realize sensitive detection of biomolecule. In this study, mesoporous silica nanospheres (MSNs)-assisted split-type PEC aptasensor with in situ generation of Bi₂S₃ was proposed to achieve reliable detection of prostate-specific antigen (PSA). To be more specific, this bioresponsive release system will release large amounts of Na₂S by the reaction between PSA and aptamer that capped Na₂S-loading MSNs. Next, the Na₂S reacts with Bi to yield BiOI/BiOBr/Bi₂S₃ composite, which leads to an alteration in the electron-hole transfer pathway of the photoelectric material and a decrease in the response. As the PSA concentration increases, more Na₂S can be released and lower photocurrent is obtained. The linear range under the optimal experimental conditions is 10 pg·mL^-1-1 μg⋅mL^-1, and the detection limit is 1.23 pg⋅mL^-1, which has satisfactory stability and anti-interference.