Photocatalytic activity of carbon nanotube/Ag3PO4 hybrid from first-principles study

Pure silver phosphate is an indirect semiconductor which has super photooxidative capabilities under visible light irradiation. Also, it can be used as a photocatalyst due to the rapid recombination of electron–hole pairs. The low structural stability of pure Ag3PO4 is the major factor militating against its use for practical applications. Under visible light irradiation, carbon nanotubes (CNTs) can increase the stability and photocatalytic activity of . This study investigated the photocatalytic activity and stability of the CNT (6, 0)/ hybrid by analyzing the geometric, electronic and optical properties with the density functional theory method. Semimetallic single-walled carbon nanotubes (SWCNT) (6, 0) may chemically or physically interact with the surface depending on its relative orientations. As its surface is exposed by SWCNT (6, 0), becomes a direct band gap semiconductor. The small band gap makes the CNT/ hybrid absorb sunlight from the ultraviolet to the infrared region.


Introduction
Ag 3 PO 4 is an excellent visible light photocatalyst with high photooxidative capability [1]. It is used to degrade organic contaminants and serves as a photofunctional material for wastewater cleaning [2]. This photocatalyst has high quantum efficiency under visible light irradiation, but, it is unstable under irradiation [3]. Numerous methods have been explored for improving and increasing the stability and photocatalytic activity of this photocatalyst. This includes the combination of Ag 3 PO 4 with different materials, including SnO 2 [4], AgX (X = Cl, Br and I) [5], TiO 2 [6], Fe 3 O 4 [7] and GO [8,9].
The Ag 3 PO 4 /g − C 3 N 4 composite was synthesized for water oxidation [10], oxygen production and pollutant deg radation [11]. Also, g − C 3 N 4 nanorod/Ag 3 PO 4 composites have been considered as one of the most effective techniques for achieving the conversion of clean and sustainable sunlight to solar fuel [12]. The bifunctional TiO 2 /Ag 3 PO 4 /graphene (GR) composites exhibited highly efficient visible light pho tocatalytic activity toward organic dye molecule degradation and showed excellent bactericidal performance [13]. Under LED illumination, Ag 3 PO 4 /Ag/graphene/graphitic carbon nitride (g − C 3 N 4 ) hetero structured materials can drive pho tocatalytic water oxidation efficiently [14].
As a result of the exceptional structure and properties of CNT, it could be used as a dopant for improving the photo catalytic degradation efficiency. The electronic properties of a CNT prepare continuous electronic states in the conduction band (CB) for donating the transferring electrons [15].
The capabilities of charge transfer of CNT can promote the excited electron in the conduction band of the semicon ductor to drift into the CNTs, thereby decreasing the ability of electron-hole pairs to recombine [16], and increasing the pho tocatalytic activity under visible light. Some composite mat erials have been proven to be effective, such as CNT/ZnO [17], CNT/C 3 N 4 [18] and CNT/TiO 2 [19,20]. CNT could be used as electron capture agents to increase the activity and stability of Ag 3 PO 4 . The photocatalytic activity of CNT/Ag 3 PO 4 has been investigated with Rhodamine B (RhB) as a model con taminant by experimental method. The experimental results showed that CNT/Ag 3 PO 4 displayed much higher photo catalytic activity than the pure Ag 3 PO 4 [16]. The chemical bonds according to covalent interaction at the interface are supposed to be charge transfer channels [21] and the Van der Waals (vdW) forces between CNTs and the semiconductor were also revealed at the interfaces [22], thereby increasing the photocatalytic activity of Ag 3 PO 4 hybrids. Various mech anisms have been suggested for increasing the photocatalytic properties of CNT/Ag 3 PO 4 hybrids. One is that in the time of photocatalysis, a highenergy photon stimulates an electron from the valence band (VB) to the CB of Ag 3 PO 4 , and the generated electrons formed in the spacecharge districts are carried into the CNTs, thereafter holes stay on Ag 3 PO 4 to take part in redox reactions [23,24]. In this work, the interaction between Ag 3 PO 4 and SWCNT (6, 0) was investigated using largescale density functional theory (DFT) computations to disclose the enhanced photocatalytic performance.

Computational method
The most stable Ag 3 PO 4 surface, cubic Ag 3 PO 4 (100) surface and metallic CNT (6, 0) were selected. The theoretical calcul ations were performed using the plane wave pseudopotential DFT method, as implemented in the CASTEP code [25]. The generalized gradient approximation (GGA) was used to describe the exchange and correlation energy of the electrons [26]. Geometry optimizations and singlepoint energy calcul ation were performed. The calculated supercells consisted of a Ag 3 PO 4 (100) surface which contained 32 O, 8 P and 24 Ag atoms and the (6, 0) tube which contained 108 atoms, length of 12.8Å in its axial direction.

Results and discussion
The experimental lattice parameter a = 6.013Å of cubic Ag 3 PO 4 with space group P4 − 3n (NO. 218) was taken to get the crystal cell [27]. By minimizing the total crystal energy, the equilibrium lattice parameter was calculated using the GGA method and the results are presented in table 1 with the experimental values. The lattice parameter includes a = 6.004Å, which is slightly overestimated less than the experimental values. There is a strong correlation between structural parameters and the experimental values.
For the noncovalent hybrid, the equilibrium distances between the CNTs and the top layer of the Ag 3 PO 4 (100) surface after optimization was calculated to be 2.450Å for CNT (6, 0) /Ag 3 PO 4 . A side view of the hybrid of CNT and the cubic Ag 3 PO 4 (100) surface is shown in figure 1. The strength of the CNT/Ag 3 PO 4 hybrid can be evaluated by their formation energy, which is defined as: where E hybrid , E CNT and E Ag3PO4 display the total energy of CNT/Ag 3 PO 4 (100), pure CNT, and clean Ag 3 PO 4 (100) surfaces, respectively. By this definition, negative E formation shows that the interface is steady and stable. The interface for mation energy was calculated to be −1.020 eV, indicating a rather strong interaction between CNTs and the Ag 3 PO 4 (100) surface, and the thermodynamic stability of this hybrid.
The density of states (DOSs) of Ag 3 PO 4 and CNT, before and after the formation of the hybrid, was calculated so as to determine the effect of the type of interfacial interaction on the electronic properties of hybrid.  Pure Ag 3 PO 4 is an indirect semiconductor with a band gap (E g ) of 2.45 eV and its CB bottom is very diffusive, causing smaller useful masses of the photogenerated electrons in pure Ag 3 PO 4 [1]. While the surface of Ag 3 PO 4 is exposed, Ag 3 PO 4 becomes a direct band gap semiconductor and E g declines to 2.15eV [28].
The DOS of pure Ag 3 PO 4 is presented in figure 2. Also, the DOS of single metallic (6, 0) CNT showed 0eV band gap, which corroborates previous studies [22]. Figure 3 shows that the DOS of (6, 0) CNT/Ag 3 PO 4 hybrid changes a little from individuals. The interaction between the metallic (6, 0) CNTs and Ag 3 PO 4 may interact via a noncovalent vdWs force. The band gap of CNT/Ag 3 PO 4 hybrid is small (0.16 < 0.3eV) mentioning that the hybrid can absorb the most sunlight, thus raising their photocatalytic activity. For the noncovalent CNT/Ag 3 PO 4 hybrid, the CB bottom is formed mainly from Ag 5s orbitals, which can be more clearly observed from the electron density distributions of the lowestunoccupied level (LUL), while the HOL is only composed of the C 2p orbitals.

Conclusions
The interaction in CNT (6, 0) and Ag 3 PO 4 hybrid depends on the nature of CNTs and their relative orientations. Metallic CNT (6, 0) may physically interact with the Ag 3 PO 4 (100). In the Ag 3 PO 4 hybrid, the band gap is small which causes the CNT/Ag 3 PO 4 hybrid to absorb sunlight from the ultra violet to the infrared region. Moreover, CNTs are not only  effective sensitizers, but are also highly active cocatalysts in hybrids. This study is useful for developing highly efficient carbonbased nanophotocatalysts. During photocatalysis, the Ag 3 PO 4 can be excited to yield photongenerated carriers and photoinduced electrons from the valence band (VB) are transferred to the conduction band (CB), leaving the holes in the VB. In hybrid of the Ag 3 PO 4 and CNT, the photo generated electrons are effectual trapped by CNT. The CNT can be increased photocatalytic activity and stability due to the fact that the introduction of CNT significantly improves the separation of photogenerated charge carriers. Thus, elec trons and holes could be usefully divided so that the photo catalytic performance of the catalyst would be modified. The optical absorption of CNT/Ag 3 PO 4 hybrids in the visible light region can be greatly increased owing to their small band gap. Fascinatingly, it was realized that the CNTs are not only effective sensitizers, but also highly active cocatalysts in hybrids.