Abstract
In this paper, the stability, electronic structure, optical properties, and work function of GaN/g-C<sub>3</sub>N<sub>4</sub> heterojunction are studied by using the first-principles plane wave ultra-soft pseudopotential method based on density functional theory. The electric field effect is also considered. The results show that the total energy for each of the three stacking modes changes little for using the two different dispersion correction methods, i.e. Tkatchenko-Scheffler and Grimme, and the total energy of mode II is the lowest, indicating that the structure of mode II is the most stable. The lattice mismatch ratio and lattice mismatch energy of GaN/g-C<sub>3</sub>N<sub>4</sub> van der Waals heterojunction are very low, indicating that the heterojunction has good stability. The heterojunction retains the basic electronic properties of GaN and g-C<sub>3</sub>N<sub>4</sub> to a great extent and can be used as a direct bandgap semiconductor material. It can be known from the work function and differential charge diagram that the charge on the heterojunction interface is transferred from GaN to g-C<sub>3</sub>N<sub>4</sub>, and a built-in electric field orientating g-C<sub>3</sub>N<sub>4</sub> from GaN is formed at the interface. The built-in electric field of the heterojunction can effectively separate the photogenerated electron-hole pairs, which is conducive to improving the photocatalytic capability of the system. Further analysis shows that the applied electric field reduces the bandgap of GaN/g-C<sub>3</sub>N<sub>4</sub> heterostructure to varying degrees. It makes it easier for electrons to transit from valence band to conduction band, which is conducive to improving the photocatalytic activity of the system. In addition, when the applied electric field is –0.6 V/Å and 0.5 V/Å separately, the semiconductor metal phase transition occurs in the heterojunction. When the applied electric field is higher than 0.3 V/Å and lower than –0.4 V/Å, in the energy band arrangement of the heterojunction there occurs the transition from type I to type II. This can better realize the separation of photogenerated electron-hole pairs and further improve the photocatalytic capactivity of the system. Therefore, the construction of heterojunction and application of external electric field proposed in this work constitute an effective means to improve the photocatalytic activity of the system.
Highlights
The results show that the total energy of the three stacking modes changes little with the two dispersion correction methods, and
4 3118 [4]Fu J W, Xu Q L, Low J X, Jiang C J, Yu J G 2019 A Appl
Summary
本文采用基于密度泛函理论的第一性原理平面波超软赝势方法研究了 GaN/g-C3N4 异质结的稳定性、电子结构、光学性质及功函数,同时考虑了电场效 应。结果表明:GaN/g-C3N4 范德瓦尔斯异质结的晶格失配率(0.9%)和晶格失配 能极低(−1.230 meV/Å2),说明该异质结稳定性很好,且该异质结在很大程度上 保留了 GaN 和 g-C3N4 的基本电子性质,可作为直接带隙半导体材料。同时, GaN/g-C3N4 异质结在界面处形成了从 GaN 指向 g-C3N4 的内建电场,使得光生电 子-空穴对可以有效分离,这有利于提高体系的光催化能力。进一步分析可知,外 加电场使 GaN/g-C3N4 异质结的禁带宽度有着不同程度的减小,使得电子从价带 跃迁至导带更加容易,有利于提高体系的光催化活性;此外,当外加电场高于 0.3 V/Å 以及低于-0.4 V/Å 时,异质结的能带排列由 I 型向 II 型过渡,更好的实现光 生电子-空穴对的分离,进一步提高体系的光催化活性。因此,本文提出的构建异 质结及施加外电场是提高体系光催化活性的有效手段。 关键词:电子结构,光学性质,功函数,外电场 PACS:73.40.Lq, 71.20.-b, 31.15.es, 36.20.Kd 因此,本文利用第一性原理的方法构建并计算了 GaN/g-C3N4 异质结,探讨 了 GaN/g-C3N4 异质结能否弥补 g-C3N4 自身的局限性,并分析该异质结的光催化 活性;研究了外加电场能否进一步提高异质结的催化效率,以期能够制备出稳定 高效的光催化剂材料,为相关的实验制备提供理论参考。 为了进一步研究 GaN/g-C3N4 异质结的结构稳定性,本研究计算了不同层间 距(distance)下的界面结合能,如图 2(b)所示。结合能( Ecoh )[22]可表示为 Ecoh ET (GaN / g-C3N4 ) ET (g-C3N4 ) ET (GaN ) 图 2 (a)GaN/g-C3N4 异质结的三种堆垛模式俯视图以及采用 TS 和 Grimme 色
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