Full Length ArticleTheoretical investigation of the water splitting photocatalytic properties of pristine, Nb and V doped, and Nb-V co-doped (1 1 1) TaON nanosheets
Graphical abstract
Introduction
Optimal energy supply and consumption are one of the most important human issues today. Fossil fuels such as coal, natural gases, and oil derivations are the usual sources of energy in the world. These energy sources are declining and will eventually run out over the next few decades. Also, Fossil fuel consumption is associated with air pollution and an increase in greenhouse effects. Therefore, finding clean and renewable energy sources to survive on Earth is inevitable. One of the great alternatives to fossil fuels is hydrogen gas (H2) which its combustion produces water. There are different methods for H2 production including natural gas reforming/gasification, electrolysis, renewable liquid reforming, fermentation, and water splitting [1], [2], [3], [4], [5]. Water splitting is an environmentally-friendly method for producing H2 which can be accomplished as photobiological or photoelectrochemical [6]. In the photoelectrochemical water splitting, the H2 is produced from water using special semiconductor as catalyst and energy from sunlight via two half-reactions, the water oxidation and the hydrogen reduction [7]:
Water oxidation: 2H2O (l) → O2 (g) + 4H+ (aq) + 4e− ΔE° = −5.67 eV vs. vacuum level
Hydrogen reduction: 4H+ (aq) + 4e− → 2H2 (g) ΔE0 = − 4.44 eV vs. vacuum level
The VB and CB edge energy of a suitable photocatalyst for water splitting should be lower and higher than the water oxidation potential and hydrogen reduction potential, respectively. In this condition, the bandgap energy of an appropriate photocatalysis must be larger than 1.23 eV [7].
In 2002, tantalum oxynitride (TaON) was introduced by Hitoki et al. [8] as a promised photocatalyst for water splitting because of its suitable bandgap (2.5 eV [9]) and its band edges (−6.6 and −4.1 eV for VB and CB, respectively [9]). The stable polymorph of TaON (β-TaON) with monoclinic structure, was discovered by Brauer and Weidlein in 1965 during the synthesis of Ta3N5 from Ta2O5 [10]. Armytage and Fender determined the structure of β-TaON using a powder neutron diffraction method [11]. In 2001, Fang et al. calculated the band structure of β-TaON via the density functional theory (DFT) [12]. The values of 1.8 and 2.1 eV were obtained for the bandgap of β-TaON via molecular dynamic pseudopotential approach using the VASP code and the Full Potential Linearized Augmented Plane Waves method using the WIEN97 code, respectively in their work [8]. Also, they estimated the experimental value of bandgap of β-TaON to be more than 2.1 eV based on measured band gaps of Ta3N5 and Ta2O5. In 2003, Wang-Jae Chun et al. measured the values of bandgap, VB edge, and CB edge energies of β-TaON (2.5, −6.6 and −4.1 eV, respectively) using ultraviolet photoelectron spectroscopy (UPS) and electrochemical analyses [9].
There are several studies on the investigation of the abilities and limitations of β-TaON as a water-splitting photocatalyst in literature [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. Hara et al. showed that adding Ru co-catalyst to TaON enhances the H2 evolution [13]. The hybridization of TaON and WO3 catalysts for water splitting process was studied by Abe et al. [16]. In this study, the H2O/O2 and H+/H2 half-reactions occurred on the WO3 and TaON catalysts within an IO3−/I− shuttle redox-mediated system. Kazuhiko et al. enhanced the H2 evolution on the TaON photocatalyst by modifying TaON with monoclinic ZrO2 [17]. For enhancing the photocatalytic efficiency of TaON, Kim et al. modified TaON with CaFe2O2 to form a heterojunction photoanode [18]. Loading Ni(OH)2 cocatalyst on TaON enhances the photocatalytic efficiency for H2 evaluation more than two times compared to bare TaON [19]. Pei et al. investigated the enhancement of photocatalytic efficiency of H2 evolution using the design of TaON/Ta3N5 heterojunction photoanode [20]. Despite abilities, TaON undergoes limitations such as self-oxidative deactivation and conversion of TaON to Ta3N5 [21], [23]. In the other study, Abe et al. performed a highly stable water splitting by dispersing CoOx nanoparticles as cocatalyst on the TaON [21] and improved the stability of CoOx/TaON cophotocatalyst by modifying the TaON surface with TiO2 [22].
In addition to experimental works, there are some theoretical studies related to the structures and band structures of TaON photocatalyst in literature [23], [24], [25], [26]. Reshak used modified Becke-Johnson potential to calculate the band structure and optical properties of monoclinic TaON [24] and obtained the value of 2.5 eV for the bandgap of TaON in agreement with the experimental value. The structure and optoelectronic properties of TaON and Ta3N5 were studied using the DFT calculations employing PBE and HSE06 functionals in the VASP package by Nurlaela et al. [25]. They obtained the band gap values of 3.0 and 2.2 eV for TaON and Ta3N5, respectively. Cui and Jiang studied the structure, stability, and band structure of (1 0 0) and (1 1 1) surfaces of β-TaON using G0W0 approximation and compared with the available experimental values [26]. They showed that (1 1 1) surface of TaON is suitable photocatalyst for water splitting whereas (1 0 0) surface can be used as n-type photocatalyst because of its CB edge energy. Also, Habib Ullah et al. compared the photocatalytic abilities of (0 1 0) and (0 0 1) surfaces of TaON for water splitting [23]. They found that the (0 1 0) surface of β-TaON can be used as water splitting photocatalyst, however, the bandgap value of (0 1 0) surface is more than that of bulky β-TaON. The bandgap value of (0 0 1) surface obtained to be less than that of (0 1 0) surface and bulky β-TaON (about 2.1 eV), but the VB level of energy of (0 0 1) is not suitable for water oxidation half-reaction. Therefore, the (0 0 1) surface can be used as a p-type photocatalyst.
In the present study, we have performed the DFT calculations employing the PBE-U method to investigate the photocatalytic capability of the (1 1 1) nanosheet of β-TaON and its V and Nb doping through the calculation of their structures and band structures.
The DFT calculations were performed within Perdew–Burke–Ernzerhof (PBE) scheme [27] of generalized gradient approximation (GGA) [28] using quantum SPERESSO open-source code [29] to obtain the structures and band structures of the pure and, V and Nb doped (1 1 1) TaON nanosheets. Since the PBE scheme usually underestimates the bandgap energy of semiconductors, so the PBE method was corrected using Hubbard coulombic parameters of U for each atom (PBE-U) [30]. The following equations were used to determined Hubbard parameters of U for Ta+5, V+5, Nb+5, O−2, and N−3 ions [31].where the single point energies of M+x, O−x, and N−x were obtained by the CCSD(T) level of theory accompanied with SDD basis set using the Gaussian 09 (G09) quantum package [32].
Surfaces of 111 of β-TaON with a thickness of 5.53 Å were constructed and separated by 20 Å vacuum space to model (1 1 1) nanosheet of TaON. The Brillouin zone of bulky and (1 1 1) nanosheet of β-TaON were described by 6*6*6 and 5*5*1 k-point samples, respectively. Also for comparing, (1 0 0) and (1 0 1) nanosheets of TaON were constructed with a primitive thickness of 9.88 and 6.60 Å, respectively. The convergence energy threshold and wave plan cutoff were set to be 7.35 × 10−7 and 70 Ry (10−5 and 952 eV), respectively, for all calculations. Moreover, the structure relaxations were performed through a maximum force of 10−3 Ry/Å (1.36 × 10−2 eV/Å) on each atom.
To estimate the thermal stabilities of pristine, Nb and V doped, and Nb-V co-doped TaON nanosheet, their formation energies (Eform) were calculated by following calculation [33], [34]:where E(structure) is the energy obulky TaON or pristine or doped TaON nanosheet per TaON formula unit, ηM is the energy per M atom of reference phase and nM is the portian of M atom in TaON formula unit. (bulky α-Ta,α-Nb and α-V were used as reference phases for Ta, Nb and V atoms, whereas ηN and ηO are the half of molecular energy of the N2 and O2 in their ground electronic state).
Section snippets
Determination of Hubbard parameters of U
The calculated values of Uion of Ta+5, Nb+5, V+5, N−3, and O-2 have been reported in Table 1. Notably, the values of Uion cannot be used directly as Hubbard parameters, because the optical dielectric constant affects the atomic coulombic correction interaction in the solid phase [31]. The Hubbard parameter of U is defined as U = uUion where the constant of u (0 < u < 1) is estimated by comparing the calculated bandgap energy and lattice parameters of bulky β-TaON with corresponding experimental
Conclusion
Based on XRD data, the (1 1 1) surface is the most important surface of the β-TaON. The comparison of (1 0 0), (1 0 1), and (1 1 1) TaON nanosheets with each other showed that the photocatalytic properties of (1 1 1) TaON nanosheet is similar to those of β-TaON. The calculated PDOS of (1 1 1) TaON nanosheet depicted that the N(2P) and Ta(5d) orbitals have the main contribution in the VB and CB edges of this nanosheet, respectively. It was observed that the replacement of Ta ions with Nb and V ions mainly
CRediT authorship contribution statement
Hamidreza Jouypazadeh: Data curation, Formal analysis, Writing - original draft, Visualization. Hossein Farrokhpour: Supervision, Conceptualization, Writing - review & editing, Investigation, Validation. Mohamad Mohsen Momeni: Conceptualization, Investigation, Validation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors acknowledge from Iran National Science Foundation (INSF) for supporting this research (Grant Number: 97001344). Also, the authors grateful to the Sheikh Bahaei National High Performance Computing Center (SBNHPCC) in Isfahan University of Technology (IUT) for providing computing facilities and time.
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