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Enhanced visible-light photocatalytic activity of a g-C3N4/BiVO4 nanocomposite: a first-principles study† Jihua Zhang,ab Fengzhu Ren,b Mingsen Deng*ac and Yuanxu Wang*ab The structural, electronic, and optical properties of a g-C3N4(001)/BiVO4(010) nanocomposite have been investigated using first-principles calculations. The results indicate that g-C3N4(001) can stably adsorb onto the BiVO4(010) surface, and it tends to form a regular wavy shape. The calculated band gap of the g-C3N4(001)/BiVO4(010) nanocomposite is narrower compared with that of BiVO4 or BiVO4(010), primarily due to the introduction of N 2p states near the Fermi level. The g-C3N4(001)/BiVO4(010) nanocomposite has a favorable type-II band alignment; thus, photoexcited electrons can be injected into the conduction band of g-C3N4(001) from the conduction band of BiVO4(010). The proper interface
Received 29th December 2014, Accepted 9th March 2015
charge distribution facilitates carrier separation in the g-C3N4(001)/BiVO4(010) interface region. The
DOI: 10.1039/c4cp06089j
absorption coefficients indicate an obvious redshift of the absorption edge, which is in good agreement with
electron injection and carrier separation can prevent the recombination of electron–hole pairs. The calculated the experimental results. Our calculation results suggest that the g-C3N4(001)/BiVO4(010) nanocomposite has
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significant advantages for visible-light photocatalysis.
1. Introduction Bismuth vanadate (BiVO4) is one of the most promising photocatalytic materials for water oxidation1–5 due to its band gap in the visible light region, material abundance, low cost, and high stability.6–9 Three different phases of BiVO4 have been observed: tetragonal zircon (tz-), tetragonal scheelite (ts-), and monoclinic scheelite (ms-).10 Among them, ms-BiVO4 is the most common phase under ambient conditions and has been found to exhibit the highest photocatalytic activity under visible-light irradiation.11,12 Therefore, we focus on the photocatalytic properties of ms-BiVO4 in the current work. However, the typical efficiencies of pure ms-BiVO4 for water oxidation are not impressive because of excessive electron–hole recombination, poor charge transport properties, and poor water oxidation kinetics.5 In practice, these deficiencies greatly limit the applications of ms-BiVO4. Previous attempts have been made to improve the photocatalytic and photoelectrochemical (PEC) activity of BiVO4 using the following techniques: forming nanocomposite a
Guizhou Provincial Key Laboratory of Computational Nano-Material Science, Guizhou Normal College, 115 Gaoxin Road, Guiyang, 550018, China. E-mail: [email protected], [email protected] b Institute for Computational Materials Science, School of Physics and Electronics, Henan University, Kaifeng 475004, China c GZNC-INSPUR Parallel Computing Laboratory, Guizhou Normal College, 550018, China † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cp06089j
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structures,13 controlling morphologies,14,15 doping,16 and coupling with oxygen evolution catalysts (OECs).17 Among the abovementioned strategies, coupling with a different semiconductor with matched energy levels is an effective method for suppressing the recombination of photogenerated electron–hole pairs by making charge separation and transfer possible and simultaneously extending the absorption edge.18–21 Meanwhile, graphite-phase carbon nitride (g-C3N4) exhibits a good visible-light response,22 and it has attracted a great deal of scientific interest for photocatalytic water splitting22–28 and organic pollutant degradation.29–33 Different from other organic p-conjugated materials, g-C3N4 is crystalline because of its lamellar structure, a characteristic that facilitates charge transfer.34,35 However, there are also many drawbacks for the g-C3N4 materials, which include a low specific surface area, a high recombination rate of photogenerated electron–hole pairs,36 and a strong reliance on the type and amount of surface co-catalyst.28,37 A co-catalyst can provide reduction or oxidation active sites, catalyze the surface reactions by lowering the activation energies, trap the charge carriers, and suppress the recombination of photogenerated electrons and holes.38–40 Therefore, choosing the proper co-catalyst for loading onto the light-harvesting semiconductor can lead to an improved photocatalytic performance including higher activity, easier selectivity, and longer stability. For example, g-C3N4-hybridized BiVO4 is expected to display excellent photocatalytic properties under visible-light irradiation by combining the advantages of g-C3N4 as
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well as BiVO4.41–44 In addition, the band edge position of g-C3N4 has been reported to match well with that of BiVO4.43,44 Experimentally, Dong et al.44 found that the photocatalytic activity of the g-C3N4/BiVO4 nanocomposite is almost 3.5 and 2.8 times higher than that of individual g-C3N4 and BiVO4, respectively. Recently, Li and coworkers43 successfully constructed an efficient g-C3N4/BiVO4 nanocomposite photocatalyst, which exhibited an excellent visiblelight response, fully exposed reactive sites, separated redox reaction sites as well as increased charge separation efficiency. These experimental results indicated that g-C3N4 sheets could serve as suitable co-catalysts in a composite with BiVO4, exhibiting enhanced photocatalytic performance. However, the geometric structural properties at the interface between the g-C3N4 sheets and the BiVO4 remain poorly understood. In addition, there is still a lack of fundamental understanding of the effects of the interfacial composition and the mechanisms behind the photocatalytic enhancement of the g-C3N4/BiVO4 nanocomposite. In this paper, we present a detailed investigation of the structural, electronic, and optical properties of a g-C3N4(001)/ BiVO4(010) nanocomposite. It is found that g-C3N4(001) can stably adsorb on the BiVO4(010) surface, and it tends to form a regular wavy shape. The band gap of the g-C3N4(001)/BiVO4(010) nanocomposite is narrower compared with that of g-C3N4(001) or BiVO4(010) mainly due to the introduction of N 2p states near the Fermi level. The carrier separation and electron injection of the g-C3N4(001)/BiVO4(010) nanocomposite can prevent the recombination of electron–hole pairs. The calculated absorption coefficients indicate an obvious redshift of the absorption edge, which is in good agreement with the experimental results. These results may be helpful for the design and application of a g-C3N4(001)/BiVO4(010) nanocomposite for using in visible-light photocatalytic nanoelectronic devices.
2. Computational details Our calculations for the g-C3N4/BiVO4 nanocomposite were performed using density functional theory (DFT), as implemented in the Vienna ab initio Simulation Package (VASP).45 The projectaugmented wave method was employed for the core–valence interactions and the generalized gradient approximation (GGA) of the Perdew–Burke–Ernzerhof (PBE) form was employed for the exchange–correlation function.45–49 Due to the absence of strong bonding interactions between g-C3N4 and BiVO4, weak van der Waals interactions are expected to play a large role. Because the standard PBE functional cannot describe the weak interactions well, we adopted a DFT-D3(BJ)50 with the Becke and Johnson (BJ)-damped51,52 vdW correction proposed by Grimme. In the DFT-D3(BJ) scheme, all force field parameters are gained based on the PBE functional, as given in the literature.50 The total energy (Etotal) is represented as: Etotal = EKS-DFT + EvdW,
(1)
where EKS-DFT is the conventional Kohn–Sham DFT energy and EvdW is the dispersion correction. A dipole correction53,54 was applied to make the computation converge more quickly and to
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eliminate the artificial electrostatic field between the periodic supercells. We also used DFT (GGA) + U within Dudarev’s approach.55 A U value of 2.7 eV56 was applied for the V 3d states in the BiVO4 and the g-C3N4/BiVO4 nanocomposite. While the properties of doped BiVO4 can be affected by the choice of U, these DFT + U calculations should be appropriate to draw reasonable conclusions. In fact, the hybrid exchange functional (HSE06)57 has previously been shown to seriously overestimate the band gap of g-C3N4 compared to the standard DFT functional.58 Moreover, HSE06 calculations for the g-C3N4/ BiVO4 nanocomposite are limited by their expensive computational time. The band gap of BiVO4 is 2.23 eV using the DFT + U method, which is close to the experimental result (2.4 eV)59 and is consistent with a previous theoretical study.60 This indicates that the chosen U value is sufficiently large for V 3d, and in the literature, the commonly applied U values for V 3d are in the range of 2–4 eV.56,61 Therefore, instead of HSE06, the GGA + U method has been adopted for the calculations presented here. The Kohn–Sham one-electron states were expanded in a planewave basis set up to 400 eV. A Monkhorst–Pack k-point mesh of 2 6 1 was used to perform the geometric optimizations and a 4 12 1 k-point mesh was used for the static total energy calculations.62 At the end of the structural optimization process, the residual Hellman–Feynman forces on each ion were less than 0.03 eV Å1. The imaginary part, e2(o), of the dielectric function, e(o), was calculated using the standard formulation: ð E XD Ve2 0 2 d3 k e2 ¼ knjpjkn 2 2 2p hm o 0 n;n (2) 0 f ðknÞ 1 f kn d Ekn Ekn0 ho ; where ho is the energy of the incident photon, p is the momentum operator ( h/i)(q/qx), (jkni) is the electronic wave function, and f (kn) is the Fermi function. The real part, e1(o), is ¨nig transformation. The related to e2(o) by the Kramer–Kro absorption coefficient, I(o), was derived from e1(o) and e2(o) as follows:63 1=2 pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi IðoÞ ¼ 2o e1 2 ðoÞ þ e2 2 ðoÞ e1 ðoÞ : (3)
3. Results and discussion The monoclinic BiVO4 structure was determined using careful volume optimization and atomic position relaxation with a primitive unit cell (consisting of four BiVO4 units). By optimizing the pure BiVO4 structure, we obtained the lattice parameters as follows: a = 7.2346 Å, b = 11.5892 Å, c = 5.0981 Å, and b = 134.7421. They are in good agreement with the experimental values:5,64 a = 7.2472 Å, b = 11.6972 Å, c = 5.0898 Å, and b = 134.2251. These results indicate that our calculation methods were reasonable, and the calculated results should be authentic. Here, we chose the BiVO4(010) surface as the substrate to support monolayer g-C3N4(001), as shown in Fig. 1; the
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Fig. 1 Models simulating the g-C3N4(001)/BiVO4(010) nanocomposite before geometric optimization (a) and after geometric optimization (b). The shortest distances between the N and Bi/O are 2.86 and 3.25 Å, respectively. The black, blue, green, red, and purple spheres represent C, N, V, O, and Bi atoms, respectively.
BiVO4(010) surface is a more stable surface than the other low index surfaces of BiVO4.65–67 More importantly, a 3 1 unit cell of BiVO4(010) is a rectangular cell of 15.416 Å 5.098 Å, pffiffiffi which is nicely matched with a 2 3 1 unit cell of monolayer g-C3N4(001). The surfaces were all cleaved from the optimized bulk structure and were represented by a slab model repeated periodically with a vacuum region of 15 Å between adjacent slabs. To analyze the effect of the surface state on the BiVO4(010) surface, we examined the surface energy, which is defined as follows: Esurf = {E(slab) nE(bulk)}/2A
(4)
where E(slab) and E(bulk) represent the total energies of the slab and bulk models, respectively. Additionally, n is the number of unit cells used to make the slab model, and A represents the surface area. The surface energies as a function of the slab model size are shown in Table 1. According to our results, a difference of less than 0.00032 eV Å2 in the surfaces of different layers of BiVO4(010) was noted when the slab contained up to 16 layers. The 16-layer slab model consisted of center fixed 4-layers and two surface 6-layers on each side. This indicates that the slab model, including at least six relaxed surfaces, was a good model to represent the BiVO4(010) surface. To find a balance between accuracy and efficiency, we used 10 layers of a BiVO4 slab with four bottom layers fixed at the bulk position in the interface model. In this work, we focused on the adsorption of a single C3N4 layer onto the BiVO4(010) surface because Fan et al.68 found that the effect of the number of C3N4 layers on the work function can be ignored. We calculated the work functions for the g-C3N4(001) and BiVO4(010) surfaces by aligning the Fermi level relative to the vacuum energy level, as shown in
Table 1 Calculated surface energies for a BiVO4 slab as a function of slab thickness
n
8
12
16
20
24
Esurface
0.0253207
0.023558
0.0223415
0.0226618
0.0220876
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Fig. 2 The electrostatic potentials for (a) the g-C3N4(001) surface, (b) the BiVO4(010) surface, and (c) the g-C3N4(001)/ BiVO4(010) nanocomposite. The red and green dashed lines represent the vacuum level (EO) and the Fermi level (EF), respectively. The g-C3N4(001), BiVO4(010), and the g-C3N4(001)/ BiVO4(010) nanocomposite geometry structure models are shown in the insets; is the work function. The labeling of the atoms is the same as in Fig. 1.
Fig. 2a and b. The work functions for the g-C3N4(001), BiVO4(010), and g-C3N4(001)/BiVO4(010) nanocomposite were found to be 4.07, 6.78 and 6.0 eV, respectively. The work functions for the g-C3N4(001) and BiVO4(010) are consistent with previous theoretical calculations.68 All types of nonequivalent structures of the g-C3N4(001)/ BiVO4(010) nanocomposite are examined according to the different symmetries. The nonequivalent structures of the gC3N4(001)/BiVO4(010) nanocomposite before and after geometric optimization are shown in Fig. 1 and Fig. S1, ESI.† Among them, the most stable structure of the g-C3N4(001)/ BiVO4(010) nanocomposite is shown in Fig. 1b, followed by the structure in Fig. S1c2, ESI† and the structure in Fig. S1b2, ESI;† the least stable is shown in Fig. S1a2, ESI.† These results indicate that g-C3N4(001) that adsorbs onto the BiVO4(010) surface formed a regular wavy shape.
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The interface adhesion energy was used to judge the stability of the system and was defined as:
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E(ad) = E(hetero) E[g-C3N4(001)] E[BiVO4(010)],
(5)
where E(hetero), E[g-C3N4(001)], and E[BiVO4(010)] represent the total energies of the relaxed g-C3N4(001)/BiVO4(010) nanocomposite, pure g-C3N4(001), and pure BiVO4(010) surfaces, respectively. If the value of the adhesion energy is negative, the interface is stable. The adhesion energy of the g-C3N4(001)/ BiVO4(010) nanocomposite was calculated to be 0.197 eV Å2, which indicates that this interface was stable. After geometric optimization, the g-C3N4(001) surface showed obvious distortion, visible in Fig. 1b, caused by the interaction between the g-C3N4(001) surface and the BiVO4(010) surface, which is similar to the results observed for a g-C3N4(001)/ ZnWO4(010) nanocomposite.69 The corresponding buckling distance (h1) is approximately 2.08 Å. The vertical separation (h2) between the g-C3N4(001) sheet and the BiVO4(010) surface was predicted to be 1.85 Å. Even though h2 is smaller than a traditional vdW interaction distance,70 which is a typical vdW equilibrium spacing. This is because the shortest distances between the N and Bi/O are 2.86 and 3.25 Å, respectively. These results are consistent with previous results.58,69 It should be
noted that when g-C3N4(001) adsorbs onto the Zn2GeO4(110) surface,68 both physical and chemical adsorption occur. However, in the case of g-C3N4(001) adsorption onto the BiVO4(010) surface, no chemical adsorption was observed. The close but nondestructive contact between g-C3N4(001) and BiVO4(010) may be a unique characteristic for applications as photocatalysts and in solar cells.71 To clearly analyze the origin of the charge carrier migration along the interface between g-C3N4(001) and BiVO4(010), the total density of states (TDOS) and the partial density of states (PDOS) between 6 eV and 5 eV were calculated for the g-C3N4(001), the BiVO4(010) surface, and the g-C3N4(001)/ BiVO4(010) nanocomposite, as presented in Fig. 3. The highest occupied state was chosen as the Fermi energy and was set to zero as the reference. By analyzing the PDOS of the g-C3N4(001), we can see from Fig. 3a and d that the valence band maximum (VBM) of the g-C3N4(001) surface is predominantly composed of N 2p states, including some hybridization with the C 2p states, while the conduction band minimum (CBM) has nearly equal contributions from the N 2p orbitals and the C 2p orbitals. Moreover, there is a strong hybridization between the N 2p orbitals and the C 2p orbitals in the CBM, which leads to a large splitting between the bonding and anti-bonding states. The
Fig. 3 The TDOS and corresponding PDOS of the g-C3N4(001), the BiVO4(010) surface and the g-C3N4(001)/BiVO4(010) nanocomposite. The HOSCO and LUSCO represent the highest occupied surface crystal orbital and the lowest unoccupied surface crystal orbital, respectively, in Fig. 4. The Fermi level is indicated by the short dashed line.
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band gap of g-C3N4(001) is approximately 1.5 eV, which is smaller than the experimental value (2.7 eV)72 because of the well-known shortcomings of the GGA functional.73 Using the Hartree–Fock exchange mixing parameter (a = 0.175) in the HSE06 functional, we calculated that the band gap of g-C3N4(001) is 2.7 eV (Fig. S2, ESI†), which is similar to the previous report.58 Although the absolute values of the calculated band gaps for g-C3N4(001) were underestimated using the GGA functional, the relative trend and regularity of these values using the same calculation method are reliable for the comparison of different models in the current work. As seen in Fig. 3b and e, the VBM of the BiVO4(010) surface was predominantly composed of O 2p states including some hybridization with V 3d states, and the CBM mainly consisted of V 3d states, including some hybridization with O 2p states. Thus, an electron would transfer from the O 2p orbital to the V 3d orbital under light irradiation. The calculated band gap of the BiVO4(010) slab was narrower than that of the bulk BiVO4. This result is consistent with previous theoretical calculations.66 Fig. 3c and f display the TDOS and PDOS of the g-C3N4(001)/ BiVO4(010), respectively. The electronic structure of the BiVO4(010) surface can be effectively modified by introducing a layer-structured g-C3N4(001) surface. The calculated band gap of the g-C3N4(001)/BiVO4(010) nanocomposite was approximately 1.14 eV, which is significantly narrower than those of bulk BiVO4 and BiVO4(010). The VBM and CBM of the BiVO4(010) in the g-C3N4(001)/ BiVO4(010) nanocomposite moved to a lower energy region compared with that of the unabsorbed BiVO4(010). Interestingly, in the energy range from 1 to 0 eV, the bands were mainly dominated by N 2p states. These introduced states are likely responsible for the narrower band gap and, consequently, lead to a shift in the optical absorption edge of the g-C3N4(001)/ BiVO4(010) nanocomposite toward the visible-light region. Under visible-light irradiation, it is possible that the electrons in the g-C3N4 N 2p states prefer to be directly excited into the V 3d and O 2p hybrid states, resulting in an easier electronic transition from the VB to the CB in the hybrid g-C3N4(001)/BiVO4(010) nanocomposite than in bulk BiVO4 and BiVO4(010). Therefore, the incorporation of the layer-structured g-C3N4(001) is expected to improve the photocatalytic activity of BiVO4(010) by enhancing its visible-light response. Experimentally, the g-C3N4(001)/BiVO4(010) nanocomposite exhibited better photo-response under visible light than the individual g-C3N4 or BiVO4 nanomaterials.18,41–43 To explore its mechanism, the highest occupied surface crystal orbital (HOSCO) and the lowest unoccupied surface crystal orbital (LUSCO) of the g-C3N4(001)/BiVO4(010) nanocomposite were calculated and are shown in Fig. 4. The results show that the electronic densities of the HOSCO are nearly localized around the g-C3N4(001) surface with the character of the N 2p orbital, while the LUSCO orbital of the interface is composed of V 3d and O 2p orbitals. When visible light irradiates the g-C3N4(001)/ BiVO4(010) nanocomposite, the N 2p electrons in the HOSCO are directly excited into the V 3d and O 2p orbitals, constituting the major electron density component of the LUSCO; this is in agreement with the DOS analysis. This suggests that
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Fig. 4 The partial charge density of the HOSCO (a) and the LUSCO (b) for the g-C3N4(001)/BiVO4(010) nanocomposite. The isosurface value is 0.02 e Å3. The labeling of the atoms is the same as in Fig. 1.
g-C3N4(001) acts as the visible-light sensitizer in the nanocomposite, and the photogenerated electron–hole pairs are then well-separated by the interface charge transfer, leading to the enhancement of visible-light photocatalytic activity in the g-C3N4(001)/BiVO4(010) nanocomposite. In this work, we found that the EF in g-C3N4(001) and BiVO4(010) is 2.49 eV and 2.28 eV, respectively, and the EF in the g-C3N4(001)/BiVO4(010) nanocomposite is 0.62 eV. The Fermi level shift implies a redistribution of charges after a g-C3N4 monolayer adsorbs onto the BiVO4(010) surface. To investigate the charge transfer between g-C3N4 and BiVO4(010), we calculated the charge density difference of the g-C3N4(001)/ BiVO4(010) nanocomposite. As shown in Fig. 5, the threedimensional charge density difference plot with an isosurface value of 0.0008 e Å3 was obtained by subtracting the calculated electronic charges of the individual g-C3N4(001) monolayer and BiVO4(010) from that of the g-C3N4(001)/BiVO4(010) nanocomposite. The cyan and yellow regions represent charge depletion and accumulation in the space, respectively. As noted, the charge transfer occurs mainly between the top atoms of BiVO4(010) and the N atoms, while minimal charge transfer occurs from the C atom to BiVO4(010); this is consistent with the previous DOS analysis. The closer the C and N atoms are to the surface, the more that charge transfer occurs. Furthermore, the charged interface region of the g-C3N4(001)/BiVO4(010) nanocomposite is very similar to the space charge region of a
Fig. 5 The different three-dimensional charge densities for the g-C3N4(001)/ BiVO4(010) nanocomposite interface with an isovalue of 0.0008 e Å3 (from the (001) (a) and (010) (b) directions). The cyan and yellow regions represent the charge depletion and accumulation in the space, respectively. The labeling of the atoms is the same as in Fig. 1.
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hybrid g-C3N4/MoS2 nanocomposite, which may result in the effective separation of the photogenerated electron–hole pairs under its polarized field.58 With Bader charge analysis,74 we noted that there was an average charge transfer of 0.1 |e| from g-C3N4 to the top BiVO4 layer. No obvious charge transfer occurred in the second bilayer of the BiVO4 surface, implying a strong screening of BiVO4 to adsorbates. A similar screening effect was also found at the g-C3N4/ZnWO4 interface,69 but the results differ from those of the g-C3N4/Zn2GeO4 interface.68 Generally, the optical absorption properties of a photocatalytic semiconductor material are closely related to its electronic band structure, which is a very important factor affecting the photocatalytic activity.68 To determine whether visible-light absorption in the g-C3N4(001)/BiVO4(010) nanocomposite could theoretically take place, the wavelength-dependent absorption coefficients were computed and are plotted in Fig. 6. For comparison, the absorption coefficients were also calculated for BiVO4, g-C3N4(001), and BiVO4(010). Moreover, we calculated the absorption coefficients of g-C3N4(001) using the HSE 06 functional (Fig. S2(c), ESI†), which can be confirmed with Yang.58 For bulk BiVO4, one main peak existed at approximately 500 nm, while for the BiVO4(010) and the g-C3N4(001)/BiVO4(010) nanocomposite in addition to the main peak at approximately 500 nm, a main peak and an additional peak appeared at approximately 320 and 400 nm, respectively. The absorption edge for BiVO4 was calculated at approximately 557 nm (2.23 eV), which is in agreement with the experimental value of 2.4 eV.59 Additionally, the main peak of the absorption coefficients at approximately 500 nm is consistent with previous experimental reports.18,41,43,44 In the 330 to 460 nm range, the absorption coefficients of the g-C3N4(001)/BiVO4(010) nanocomposite are larger than those of the g-C3N4(001) and the BiVO4(010) surface. A red shift was observed for the absorption peak of BiVO4(010) and the g-C3N4(001)/BiVO4(010) nanocomposite compared with the bulk BiVO4, which verifies that the band gap of the complex narrowed. These results are in a good agreement with the electronic properties mentioned above and previous experimental reports.18,41,43,44
Fig. 6 Absorption spectra of the BiVO4, the BiVO4(010) surface, the g-C3N4(001) surface, and the g-C3N4(001)/BiVO4(010) nanocomposite.
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The conduction band potentials of a semiconductor at the point of zero charge can be predicted by the Mulliken electronegativity theory:75 ECB = w Ec 0.5Eg,
(6)
where, ECB is the conduction band potential, the w is the absolute electronegativity of the semiconductor (i.e., the geometric mean of the constituent atoms), Ec is the energy of the free electron on the hydrogen scale (approximately 4.5 eV),76 and Eg is the band-gap energy of the semiconductor. The valence band potential can be calculated from EVB = ECB Eg. The w value for BiVO4 is 6.04 eV.77 Hence, the ECB value of BiVO4 was calculated to be 0.34 eV, and the EVB value was estimated to be 2.74 eV. Based on the band gap positions, the positions of the conduction and the valence bands of g-C3N4 were determined to be 1.57 and 1.13 eV, respectively.18,78 Because the relationship between ENHE (NHE = normal hydrogen electrode) and EAVS (AVS = absolute vacuum scale) is EAVS = ENHE Ec, the energies of the CB edge of BiVO4 and g-C3N4 were estimated to be 4.84 eV and 3.37 eV, respectively. Based on the band gap positions, the energies of the VB edge of BiVO4 and g-C3N4 were determined to be 7.24 eV and 6.07 eV, respectively. This indicates that the valence band offset (VBO) and the conduction band offset (CBO) between g-C3N4 and BiVO4 are approximately 1.47 eV and 1.17 eV, respectively. Fig. 7 shows a schematic diagram of the band configurations of g-C3N4 and BiVO4(010) before and after contact. As shown in this figure, because the work function of BiVO4 is higher than that of g-C3N4, the electrons will flow from g-C3N4 to BiVO4 if they contact. Therefore, BiVO4 will be negatively charged and g-C3N4 will be positively charged near the interface due to electrostatic induction. When the two phases acquire an equalized Fermi level, a built-in electric field directed from g-C3N4 to the BiVO4 will be established at the same time, which can stop the charge diffusion between g-C3N4 and BiVO4. Meanwhile, the energy bands of g-C3N4 will shift downward 1.93 eV along with its Fermi level while the energy bands of BiVO4 will shift upward 0.78 eV along with its Fermi level. Consequently, the CB bottom of BiVO4 will become higher than that of g-C3N4, and the VB top of BiVO4 will be higher than that of g-C3N4. Under visiblelight irradiation, both g-C3N4 and BiVO4 absorb photons of energy greater than the corresponding band gap energy, which will excite the electrons in the VB to the CB and will leave holes in the VB. The photogenerated electrons produced by BiVO4 will be injected into the CB of g-C3N4; meanwhile, the photogenerated holes are effectively collected in the VB of g-C3N4, owing to the well-aligned straddling band structures of the g-C3N4(001)/BiVO4(010) nanocomposite at the interface. Therefore, the probability of electron–hole recombination was reduced. In addition, the presence of the built-in electric field near the interface can also facilitate the separation of photogenerated electron–hole pairs. It is shown that the g-C3N4(001)/ BiVO4(010) nanocomposite can greatly inhibit the electron–hole pair recombination and promote the separation of electron–hole pairs, resulting in the enhanced photocatalytic activities of the nanocomposite.
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Fig. 7 A schematic diagram of the band configuration and the charge separation at the interface of the g-C3N4(001)/ BiVO4(010) nanocomposite under visible-light irradiation; CBO is the conduction band offset, VBO is the valence band offset, F is the work function, EC is the bottom of the conduction band, EV is the top of the valence band, Eg is the band gap, and EF1 and EF2 are the Fermi levels of g-C3N4 and BiVO4, respectively.
4. Conclusions In summary, we calculated the energetic, electronic and optical properties of a g-C3N4(001)/BiVO4(010) nanocomposite using the density functional method. It is determined that g-C3N4(001) can stably adsorb onto the BiVO4(010) surface, and it forms a regular wavy shape. The calculated band gap of the g-C3N4(001)/BiVO4(010) nanocomposite is narrower compared with those of the BiVO4 and BiVO4(010), mainly due to the introduction of N 2p states near the Fermi level. Furthermore, the g-C3N4(001)/BiVO4(010) nanocomposite has a favorable type-II band alignment, and thus the photoexcited electron can be injected into the conduction band of BiVO4(010) from that of g-C3N4(001). The proper interface charge distribution facilitates the carrier separation at the g-C3N4(001)/BiVO4(010) interface region. The carrier separation and electron injection can prevent the recombination of electron–hole pairs. The calculated absorption coefficients indicate an obvious redshift of the absorption edge, which is in good agreement with experimental results. This work provides an important indication of how to promote the photocatalytic activity in g-C3N4(001)/BiVO4(010) nanocomposites.
Acknowledgements This work was supported by the Program for Innovative Research Team in the University of Henan Province (no. 13IRTSTHN017), the NSF from department of education of Guizhou province (no. 2114118006zx, QJTD[2013]16), the GZNC
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startup package (no. 13BS027), the Construction Project for Guizhou Provincial Key Laboratories (no. ZJ[2011]4007, ZJ[2013]4009), and the Program for Innovative Research Team of Guizhou Province (No. QKTD[2014]4021). M. S. D would also like to acknowledge the support from the Excellent Youth Scientific and Technological Talents of Guizhou Province (no. QKH-RZ[2013]01).
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Enhanced visible-light photocatalytic activity of a g-C3N4/BiVO4 nanocomposite: a first-principles study† Jihua Zhang,ab Fengzhu Ren,b Mingsen Deng*ac and Yuanxu Wang*ab The structural, electronic, and optical properties of a g-C3N4(001)/BiVO4(010) nanocomposite have been investigated using first-principles calculations. The results indicate that g-C3N4(001) can stably adsorb onto the BiVO4(010) surface, and it tends to form a regular wavy shape. The calculated band gap of the g-C3N4(001)/BiVO4(010) nanocomposite is narrower compared with that of BiVO4 or BiVO4(010), primarily due to the introduction of N 2p states near the Fermi level. The g-C3N4(001)/BiVO4(010) nanocomposite has a favorable type-II band alignment; thus, photoexcited electrons can be injected into the conduction band of g-C3N4(001) from the conduction band of BiVO4(010). The proper interface
Received 29th December 2014, Accepted 9th March 2015
charge distribution facilitates carrier separation in the g-C3N4(001)/BiVO4(010) interface region. The
DOI: 10.1039/c4cp06089j
absorption coefficients indicate an obvious redshift of the absorption edge, which is in good agreement with
electron injection and carrier separation can prevent the recombination of electron–hole pairs. The calculated the experimental results. Our calculation results suggest that the g-C3N4(001)/BiVO4(010) nanocomposite has
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significant advantages for visible-light photocatalysis.
1. Introduction Bismuth vanadate (BiVO4) is one of the most promising photocatalytic materials for water oxidation1–5 due to its band gap in the visible light region, material abundance, low cost, and high stability.6–9 Three different phases of BiVO4 have been observed: tetragonal zircon (tz-), tetragonal scheelite (ts-), and monoclinic scheelite (ms-).10 Among them, ms-BiVO4 is the most common phase under ambient conditions and has been found to exhibit the highest photocatalytic activity under visible-light irradiation.11,12 Therefore, we focus on the photocatalytic properties of ms-BiVO4 in the current work. However, the typical efficiencies of pure ms-BiVO4 for water oxidation are not impressive because of excessive electron–hole recombination, poor charge transport properties, and poor water oxidation kinetics.5 In practice, these deficiencies greatly limit the applications of ms-BiVO4. Previous attempts have been made to improve the photocatalytic and photoelectrochemical (PEC) activity of BiVO4 using the following techniques: forming nanocomposite a
Guizhou Provincial Key Laboratory of Computational Nano-Material Science, Guizhou Normal College, 115 Gaoxin Road, Guiyang, 550018, China. E-mail: [email protected], [email protected] b Institute for Computational Materials Science, School of Physics and Electronics, Henan University, Kaifeng 475004, China c GZNC-INSPUR Parallel Computing Laboratory, Guizhou Normal College, 550018, China † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cp06089j
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structures,13 controlling morphologies,14,15 doping,16 and coupling with oxygen evolution catalysts (OECs).17 Among the abovementioned strategies, coupling with a different semiconductor with matched energy levels is an effective method for suppressing the recombination of photogenerated electron–hole pairs by making charge separation and transfer possible and simultaneously extending the absorption edge.18–21 Meanwhile, graphite-phase carbon nitride (g-C3N4) exhibits a good visible-light response,22 and it has attracted a great deal of scientific interest for photocatalytic water splitting22–28 and organic pollutant degradation.29–33 Different from other organic p-conjugated materials, g-C3N4 is crystalline because of its lamellar structure, a characteristic that facilitates charge transfer.34,35 However, there are also many drawbacks for the g-C3N4 materials, which include a low specific surface area, a high recombination rate of photogenerated electron–hole pairs,36 and a strong reliance on the type and amount of surface co-catalyst.28,37 A co-catalyst can provide reduction or oxidation active sites, catalyze the surface reactions by lowering the activation energies, trap the charge carriers, and suppress the recombination of photogenerated electrons and holes.38–40 Therefore, choosing the proper co-catalyst for loading onto the light-harvesting semiconductor can lead to an improved photocatalytic performance including higher activity, easier selectivity, and longer stability. For example, g-C3N4-hybridized BiVO4 is expected to display excellent photocatalytic properties under visible-light irradiation by combining the advantages of g-C3N4 as
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well as BiVO4.41–44 In addition, the band edge position of g-C3N4 has been reported to match well with that of BiVO4.43,44 Experimentally, Dong et al.44 found that the photocatalytic activity of the g-C3N4/BiVO4 nanocomposite is almost 3.5 and 2.8 times higher than that of individual g-C3N4 and BiVO4, respectively. Recently, Li and coworkers43 successfully constructed an efficient g-C3N4/BiVO4 nanocomposite photocatalyst, which exhibited an excellent visiblelight response, fully exposed reactive sites, separated redox reaction sites as well as increased charge separation efficiency. These experimental results indicated that g-C3N4 sheets could serve as suitable co-catalysts in a composite with BiVO4, exhibiting enhanced photocatalytic performance. However, the geometric structural properties at the interface between the g-C3N4 sheets and the BiVO4 remain poorly understood. In addition, there is still a lack of fundamental understanding of the effects of the interfacial composition and the mechanisms behind the photocatalytic enhancement of the g-C3N4/BiVO4 nanocomposite. In this paper, we present a detailed investigation of the structural, electronic, and optical properties of a g-C3N4(001)/ BiVO4(010) nanocomposite. It is found that g-C3N4(001) can stably adsorb on the BiVO4(010) surface, and it tends to form a regular wavy shape. The band gap of the g-C3N4(001)/BiVO4(010) nanocomposite is narrower compared with that of g-C3N4(001) or BiVO4(010) mainly due to the introduction of N 2p states near the Fermi level. The carrier separation and electron injection of the g-C3N4(001)/BiVO4(010) nanocomposite can prevent the recombination of electron–hole pairs. The calculated absorption coefficients indicate an obvious redshift of the absorption edge, which is in good agreement with the experimental results. These results may be helpful for the design and application of a g-C3N4(001)/BiVO4(010) nanocomposite for using in visible-light photocatalytic nanoelectronic devices.
2. Computational details Our calculations for the g-C3N4/BiVO4 nanocomposite were performed using density functional theory (DFT), as implemented in the Vienna ab initio Simulation Package (VASP).45 The projectaugmented wave method was employed for the core–valence interactions and the generalized gradient approximation (GGA) of the Perdew–Burke–Ernzerhof (PBE) form was employed for the exchange–correlation function.45–49 Due to the absence of strong bonding interactions between g-C3N4 and BiVO4, weak van der Waals interactions are expected to play a large role. Because the standard PBE functional cannot describe the weak interactions well, we adopted a DFT-D3(BJ)50 with the Becke and Johnson (BJ)-damped51,52 vdW correction proposed by Grimme. In the DFT-D3(BJ) scheme, all force field parameters are gained based on the PBE functional, as given in the literature.50 The total energy (Etotal) is represented as: Etotal = EKS-DFT + EvdW,
(1)
where EKS-DFT is the conventional Kohn–Sham DFT energy and EvdW is the dispersion correction. A dipole correction53,54 was applied to make the computation converge more quickly and to
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eliminate the artificial electrostatic field between the periodic supercells. We also used DFT (GGA) + U within Dudarev’s approach.55 A U value of 2.7 eV56 was applied for the V 3d states in the BiVO4 and the g-C3N4/BiVO4 nanocomposite. While the properties of doped BiVO4 can be affected by the choice of U, these DFT + U calculations should be appropriate to draw reasonable conclusions. In fact, the hybrid exchange functional (HSE06)57 has previously been shown to seriously overestimate the band gap of g-C3N4 compared to the standard DFT functional.58 Moreover, HSE06 calculations for the g-C3N4/ BiVO4 nanocomposite are limited by their expensive computational time. The band gap of BiVO4 is 2.23 eV using the DFT + U method, which is close to the experimental result (2.4 eV)59 and is consistent with a previous theoretical study.60 This indicates that the chosen U value is sufficiently large for V 3d, and in the literature, the commonly applied U values for V 3d are in the range of 2–4 eV.56,61 Therefore, instead of HSE06, the GGA + U method has been adopted for the calculations presented here. The Kohn–Sham one-electron states were expanded in a planewave basis set up to 400 eV. A Monkhorst–Pack k-point mesh of 2 6 1 was used to perform the geometric optimizations and a 4 12 1 k-point mesh was used for the static total energy calculations.62 At the end of the structural optimization process, the residual Hellman–Feynman forces on each ion were less than 0.03 eV Å1. The imaginary part, e2(o), of the dielectric function, e(o), was calculated using the standard formulation: ð E XD Ve2 0 2 d3 k e2 ¼ knjpjkn 2 2 2p hm o 0 n;n (2) 0 f ðknÞ 1 f kn d Ekn Ekn0 ho ; where ho is the energy of the incident photon, p is the momentum operator ( h/i)(q/qx), (jkni) is the electronic wave function, and f (kn) is the Fermi function. The real part, e1(o), is ¨nig transformation. The related to e2(o) by the Kramer–Kro absorption coefficient, I(o), was derived from e1(o) and e2(o) as follows:63 1=2 pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi IðoÞ ¼ 2o e1 2 ðoÞ þ e2 2 ðoÞ e1 ðoÞ : (3)
3. Results and discussion The monoclinic BiVO4 structure was determined using careful volume optimization and atomic position relaxation with a primitive unit cell (consisting of four BiVO4 units). By optimizing the pure BiVO4 structure, we obtained the lattice parameters as follows: a = 7.2346 Å, b = 11.5892 Å, c = 5.0981 Å, and b = 134.7421. They are in good agreement with the experimental values:5,64 a = 7.2472 Å, b = 11.6972 Å, c = 5.0898 Å, and b = 134.2251. These results indicate that our calculation methods were reasonable, and the calculated results should be authentic. Here, we chose the BiVO4(010) surface as the substrate to support monolayer g-C3N4(001), as shown in Fig. 1; the
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Fig. 1 Models simulating the g-C3N4(001)/BiVO4(010) nanocomposite before geometric optimization (a) and after geometric optimization (b). The shortest distances between the N and Bi/O are 2.86 and 3.25 Å, respectively. The black, blue, green, red, and purple spheres represent C, N, V, O, and Bi atoms, respectively.
BiVO4(010) surface is a more stable surface than the other low index surfaces of BiVO4.65–67 More importantly, a 3 1 unit cell of BiVO4(010) is a rectangular cell of 15.416 Å 5.098 Å, pffiffiffi which is nicely matched with a 2 3 1 unit cell of monolayer g-C3N4(001). The surfaces were all cleaved from the optimized bulk structure and were represented by a slab model repeated periodically with a vacuum region of 15 Å between adjacent slabs. To analyze the effect of the surface state on the BiVO4(010) surface, we examined the surface energy, which is defined as follows: Esurf = {E(slab) nE(bulk)}/2A
(4)
where E(slab) and E(bulk) represent the total energies of the slab and bulk models, respectively. Additionally, n is the number of unit cells used to make the slab model, and A represents the surface area. The surface energies as a function of the slab model size are shown in Table 1. According to our results, a difference of less than 0.00032 eV Å2 in the surfaces of different layers of BiVO4(010) was noted when the slab contained up to 16 layers. The 16-layer slab model consisted of center fixed 4-layers and two surface 6-layers on each side. This indicates that the slab model, including at least six relaxed surfaces, was a good model to represent the BiVO4(010) surface. To find a balance between accuracy and efficiency, we used 10 layers of a BiVO4 slab with four bottom layers fixed at the bulk position in the interface model. In this work, we focused on the adsorption of a single C3N4 layer onto the BiVO4(010) surface because Fan et al.68 found that the effect of the number of C3N4 layers on the work function can be ignored. We calculated the work functions for the g-C3N4(001) and BiVO4(010) surfaces by aligning the Fermi level relative to the vacuum energy level, as shown in
Table 1 Calculated surface energies for a BiVO4 slab as a function of slab thickness
n
8
12
16
20
24
Esurface
0.0253207
0.023558
0.0223415
0.0226618
0.0220876
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Fig. 2 The electrostatic potentials for (a) the g-C3N4(001) surface, (b) the BiVO4(010) surface, and (c) the g-C3N4(001)/ BiVO4(010) nanocomposite. The red and green dashed lines represent the vacuum level (EO) and the Fermi level (EF), respectively. The g-C3N4(001), BiVO4(010), and the g-C3N4(001)/ BiVO4(010) nanocomposite geometry structure models are shown in the insets; is the work function. The labeling of the atoms is the same as in Fig. 1.
Fig. 2a and b. The work functions for the g-C3N4(001), BiVO4(010), and g-C3N4(001)/BiVO4(010) nanocomposite were found to be 4.07, 6.78 and 6.0 eV, respectively. The work functions for the g-C3N4(001) and BiVO4(010) are consistent with previous theoretical calculations.68 All types of nonequivalent structures of the g-C3N4(001)/ BiVO4(010) nanocomposite are examined according to the different symmetries. The nonequivalent structures of the gC3N4(001)/BiVO4(010) nanocomposite before and after geometric optimization are shown in Fig. 1 and Fig. S1, ESI.† Among them, the most stable structure of the g-C3N4(001)/ BiVO4(010) nanocomposite is shown in Fig. 1b, followed by the structure in Fig. S1c2, ESI† and the structure in Fig. S1b2, ESI;† the least stable is shown in Fig. S1a2, ESI.† These results indicate that g-C3N4(001) that adsorbs onto the BiVO4(010) surface formed a regular wavy shape.
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The interface adhesion energy was used to judge the stability of the system and was defined as:
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E(ad) = E(hetero) E[g-C3N4(001)] E[BiVO4(010)],
(5)
where E(hetero), E[g-C3N4(001)], and E[BiVO4(010)] represent the total energies of the relaxed g-C3N4(001)/BiVO4(010) nanocomposite, pure g-C3N4(001), and pure BiVO4(010) surfaces, respectively. If the value of the adhesion energy is negative, the interface is stable. The adhesion energy of the g-C3N4(001)/ BiVO4(010) nanocomposite was calculated to be 0.197 eV Å2, which indicates that this interface was stable. After geometric optimization, the g-C3N4(001) surface showed obvious distortion, visible in Fig. 1b, caused by the interaction between the g-C3N4(001) surface and the BiVO4(010) surface, which is similar to the results observed for a g-C3N4(001)/ ZnWO4(010) nanocomposite.69 The corresponding buckling distance (h1) is approximately 2.08 Å. The vertical separation (h2) between the g-C3N4(001) sheet and the BiVO4(010) surface was predicted to be 1.85 Å. Even though h2 is smaller than a traditional vdW interaction distance,70 which is a typical vdW equilibrium spacing. This is because the shortest distances between the N and Bi/O are 2.86 and 3.25 Å, respectively. These results are consistent with previous results.58,69 It should be
noted that when g-C3N4(001) adsorbs onto the Zn2GeO4(110) surface,68 both physical and chemical adsorption occur. However, in the case of g-C3N4(001) adsorption onto the BiVO4(010) surface, no chemical adsorption was observed. The close but nondestructive contact between g-C3N4(001) and BiVO4(010) may be a unique characteristic for applications as photocatalysts and in solar cells.71 To clearly analyze the origin of the charge carrier migration along the interface between g-C3N4(001) and BiVO4(010), the total density of states (TDOS) and the partial density of states (PDOS) between 6 eV and 5 eV were calculated for the g-C3N4(001), the BiVO4(010) surface, and the g-C3N4(001)/ BiVO4(010) nanocomposite, as presented in Fig. 3. The highest occupied state was chosen as the Fermi energy and was set to zero as the reference. By analyzing the PDOS of the g-C3N4(001), we can see from Fig. 3a and d that the valence band maximum (VBM) of the g-C3N4(001) surface is predominantly composed of N 2p states, including some hybridization with the C 2p states, while the conduction band minimum (CBM) has nearly equal contributions from the N 2p orbitals and the C 2p orbitals. Moreover, there is a strong hybridization between the N 2p orbitals and the C 2p orbitals in the CBM, which leads to a large splitting between the bonding and anti-bonding states. The
Fig. 3 The TDOS and corresponding PDOS of the g-C3N4(001), the BiVO4(010) surface and the g-C3N4(001)/BiVO4(010) nanocomposite. The HOSCO and LUSCO represent the highest occupied surface crystal orbital and the lowest unoccupied surface crystal orbital, respectively, in Fig. 4. The Fermi level is indicated by the short dashed line.
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band gap of g-C3N4(001) is approximately 1.5 eV, which is smaller than the experimental value (2.7 eV)72 because of the well-known shortcomings of the GGA functional.73 Using the Hartree–Fock exchange mixing parameter (a = 0.175) in the HSE06 functional, we calculated that the band gap of g-C3N4(001) is 2.7 eV (Fig. S2, ESI†), which is similar to the previous report.58 Although the absolute values of the calculated band gaps for g-C3N4(001) were underestimated using the GGA functional, the relative trend and regularity of these values using the same calculation method are reliable for the comparison of different models in the current work. As seen in Fig. 3b and e, the VBM of the BiVO4(010) surface was predominantly composed of O 2p states including some hybridization with V 3d states, and the CBM mainly consisted of V 3d states, including some hybridization with O 2p states. Thus, an electron would transfer from the O 2p orbital to the V 3d orbital under light irradiation. The calculated band gap of the BiVO4(010) slab was narrower than that of the bulk BiVO4. This result is consistent with previous theoretical calculations.66 Fig. 3c and f display the TDOS and PDOS of the g-C3N4(001)/ BiVO4(010), respectively. The electronic structure of the BiVO4(010) surface can be effectively modified by introducing a layer-structured g-C3N4(001) surface. The calculated band gap of the g-C3N4(001)/BiVO4(010) nanocomposite was approximately 1.14 eV, which is significantly narrower than those of bulk BiVO4 and BiVO4(010). The VBM and CBM of the BiVO4(010) in the g-C3N4(001)/ BiVO4(010) nanocomposite moved to a lower energy region compared with that of the unabsorbed BiVO4(010). Interestingly, in the energy range from 1 to 0 eV, the bands were mainly dominated by N 2p states. These introduced states are likely responsible for the narrower band gap and, consequently, lead to a shift in the optical absorption edge of the g-C3N4(001)/ BiVO4(010) nanocomposite toward the visible-light region. Under visible-light irradiation, it is possible that the electrons in the g-C3N4 N 2p states prefer to be directly excited into the V 3d and O 2p hybrid states, resulting in an easier electronic transition from the VB to the CB in the hybrid g-C3N4(001)/BiVO4(010) nanocomposite than in bulk BiVO4 and BiVO4(010). Therefore, the incorporation of the layer-structured g-C3N4(001) is expected to improve the photocatalytic activity of BiVO4(010) by enhancing its visible-light response. Experimentally, the g-C3N4(001)/BiVO4(010) nanocomposite exhibited better photo-response under visible light than the individual g-C3N4 or BiVO4 nanomaterials.18,41–43 To explore its mechanism, the highest occupied surface crystal orbital (HOSCO) and the lowest unoccupied surface crystal orbital (LUSCO) of the g-C3N4(001)/BiVO4(010) nanocomposite were calculated and are shown in Fig. 4. The results show that the electronic densities of the HOSCO are nearly localized around the g-C3N4(001) surface with the character of the N 2p orbital, while the LUSCO orbital of the interface is composed of V 3d and O 2p orbitals. When visible light irradiates the g-C3N4(001)/ BiVO4(010) nanocomposite, the N 2p electrons in the HOSCO are directly excited into the V 3d and O 2p orbitals, constituting the major electron density component of the LUSCO; this is in agreement with the DOS analysis. This suggests that
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Fig. 4 The partial charge density of the HOSCO (a) and the LUSCO (b) for the g-C3N4(001)/BiVO4(010) nanocomposite. The isosurface value is 0.02 e Å3. The labeling of the atoms is the same as in Fig. 1.
g-C3N4(001) acts as the visible-light sensitizer in the nanocomposite, and the photogenerated electron–hole pairs are then well-separated by the interface charge transfer, leading to the enhancement of visible-light photocatalytic activity in the g-C3N4(001)/BiVO4(010) nanocomposite. In this work, we found that the EF in g-C3N4(001) and BiVO4(010) is 2.49 eV and 2.28 eV, respectively, and the EF in the g-C3N4(001)/BiVO4(010) nanocomposite is 0.62 eV. The Fermi level shift implies a redistribution of charges after a g-C3N4 monolayer adsorbs onto the BiVO4(010) surface. To investigate the charge transfer between g-C3N4 and BiVO4(010), we calculated the charge density difference of the g-C3N4(001)/ BiVO4(010) nanocomposite. As shown in Fig. 5, the threedimensional charge density difference plot with an isosurface value of 0.0008 e Å3 was obtained by subtracting the calculated electronic charges of the individual g-C3N4(001) monolayer and BiVO4(010) from that of the g-C3N4(001)/BiVO4(010) nanocomposite. The cyan and yellow regions represent charge depletion and accumulation in the space, respectively. As noted, the charge transfer occurs mainly between the top atoms of BiVO4(010) and the N atoms, while minimal charge transfer occurs from the C atom to BiVO4(010); this is consistent with the previous DOS analysis. The closer the C and N atoms are to the surface, the more that charge transfer occurs. Furthermore, the charged interface region of the g-C3N4(001)/BiVO4(010) nanocomposite is very similar to the space charge region of a
Fig. 5 The different three-dimensional charge densities for the g-C3N4(001)/ BiVO4(010) nanocomposite interface with an isovalue of 0.0008 e Å3 (from the (001) (a) and (010) (b) directions). The cyan and yellow regions represent the charge depletion and accumulation in the space, respectively. The labeling of the atoms is the same as in Fig. 1.
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hybrid g-C3N4/MoS2 nanocomposite, which may result in the effective separation of the photogenerated electron–hole pairs under its polarized field.58 With Bader charge analysis,74 we noted that there was an average charge transfer of 0.1 |e| from g-C3N4 to the top BiVO4 layer. No obvious charge transfer occurred in the second bilayer of the BiVO4 surface, implying a strong screening of BiVO4 to adsorbates. A similar screening effect was also found at the g-C3N4/ZnWO4 interface,69 but the results differ from those of the g-C3N4/Zn2GeO4 interface.68 Generally, the optical absorption properties of a photocatalytic semiconductor material are closely related to its electronic band structure, which is a very important factor affecting the photocatalytic activity.68 To determine whether visible-light absorption in the g-C3N4(001)/BiVO4(010) nanocomposite could theoretically take place, the wavelength-dependent absorption coefficients were computed and are plotted in Fig. 6. For comparison, the absorption coefficients were also calculated for BiVO4, g-C3N4(001), and BiVO4(010). Moreover, we calculated the absorption coefficients of g-C3N4(001) using the HSE 06 functional (Fig. S2(c), ESI†), which can be confirmed with Yang.58 For bulk BiVO4, one main peak existed at approximately 500 nm, while for the BiVO4(010) and the g-C3N4(001)/BiVO4(010) nanocomposite in addition to the main peak at approximately 500 nm, a main peak and an additional peak appeared at approximately 320 and 400 nm, respectively. The absorption edge for BiVO4 was calculated at approximately 557 nm (2.23 eV), which is in agreement with the experimental value of 2.4 eV.59 Additionally, the main peak of the absorption coefficients at approximately 500 nm is consistent with previous experimental reports.18,41,43,44 In the 330 to 460 nm range, the absorption coefficients of the g-C3N4(001)/BiVO4(010) nanocomposite are larger than those of the g-C3N4(001) and the BiVO4(010) surface. A red shift was observed for the absorption peak of BiVO4(010) and the g-C3N4(001)/BiVO4(010) nanocomposite compared with the bulk BiVO4, which verifies that the band gap of the complex narrowed. These results are in a good agreement with the electronic properties mentioned above and previous experimental reports.18,41,43,44
Fig. 6 Absorption spectra of the BiVO4, the BiVO4(010) surface, the g-C3N4(001) surface, and the g-C3N4(001)/BiVO4(010) nanocomposite.
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The conduction band potentials of a semiconductor at the point of zero charge can be predicted by the Mulliken electronegativity theory:75 ECB = w Ec 0.5Eg,
(6)
where, ECB is the conduction band potential, the w is the absolute electronegativity of the semiconductor (i.e., the geometric mean of the constituent atoms), Ec is the energy of the free electron on the hydrogen scale (approximately 4.5 eV),76 and Eg is the band-gap energy of the semiconductor. The valence band potential can be calculated from EVB = ECB Eg. The w value for BiVO4 is 6.04 eV.77 Hence, the ECB value of BiVO4 was calculated to be 0.34 eV, and the EVB value was estimated to be 2.74 eV. Based on the band gap positions, the positions of the conduction and the valence bands of g-C3N4 were determined to be 1.57 and 1.13 eV, respectively.18,78 Because the relationship between ENHE (NHE = normal hydrogen electrode) and EAVS (AVS = absolute vacuum scale) is EAVS = ENHE Ec, the energies of the CB edge of BiVO4 and g-C3N4 were estimated to be 4.84 eV and 3.37 eV, respectively. Based on the band gap positions, the energies of the VB edge of BiVO4 and g-C3N4 were determined to be 7.24 eV and 6.07 eV, respectively. This indicates that the valence band offset (VBO) and the conduction band offset (CBO) between g-C3N4 and BiVO4 are approximately 1.47 eV and 1.17 eV, respectively. Fig. 7 shows a schematic diagram of the band configurations of g-C3N4 and BiVO4(010) before and after contact. As shown in this figure, because the work function of BiVO4 is higher than that of g-C3N4, the electrons will flow from g-C3N4 to BiVO4 if they contact. Therefore, BiVO4 will be negatively charged and g-C3N4 will be positively charged near the interface due to electrostatic induction. When the two phases acquire an equalized Fermi level, a built-in electric field directed from g-C3N4 to the BiVO4 will be established at the same time, which can stop the charge diffusion between g-C3N4 and BiVO4. Meanwhile, the energy bands of g-C3N4 will shift downward 1.93 eV along with its Fermi level while the energy bands of BiVO4 will shift upward 0.78 eV along with its Fermi level. Consequently, the CB bottom of BiVO4 will become higher than that of g-C3N4, and the VB top of BiVO4 will be higher than that of g-C3N4. Under visiblelight irradiation, both g-C3N4 and BiVO4 absorb photons of energy greater than the corresponding band gap energy, which will excite the electrons in the VB to the CB and will leave holes in the VB. The photogenerated electrons produced by BiVO4 will be injected into the CB of g-C3N4; meanwhile, the photogenerated holes are effectively collected in the VB of g-C3N4, owing to the well-aligned straddling band structures of the g-C3N4(001)/BiVO4(010) nanocomposite at the interface. Therefore, the probability of electron–hole recombination was reduced. In addition, the presence of the built-in electric field near the interface can also facilitate the separation of photogenerated electron–hole pairs. It is shown that the g-C3N4(001)/ BiVO4(010) nanocomposite can greatly inhibit the electron–hole pair recombination and promote the separation of electron–hole pairs, resulting in the enhanced photocatalytic activities of the nanocomposite.
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Fig. 7 A schematic diagram of the band configuration and the charge separation at the interface of the g-C3N4(001)/ BiVO4(010) nanocomposite under visible-light irradiation; CBO is the conduction band offset, VBO is the valence band offset, F is the work function, EC is the bottom of the conduction band, EV is the top of the valence band, Eg is the band gap, and EF1 and EF2 are the Fermi levels of g-C3N4 and BiVO4, respectively.
4. Conclusions In summary, we calculated the energetic, electronic and optical properties of a g-C3N4(001)/BiVO4(010) nanocomposite using the density functional method. It is determined that g-C3N4(001) can stably adsorb onto the BiVO4(010) surface, and it forms a regular wavy shape. The calculated band gap of the g-C3N4(001)/BiVO4(010) nanocomposite is narrower compared with those of the BiVO4 and BiVO4(010), mainly due to the introduction of N 2p states near the Fermi level. Furthermore, the g-C3N4(001)/BiVO4(010) nanocomposite has a favorable type-II band alignment, and thus the photoexcited electron can be injected into the conduction band of BiVO4(010) from that of g-C3N4(001). The proper interface charge distribution facilitates the carrier separation at the g-C3N4(001)/BiVO4(010) interface region. The carrier separation and electron injection can prevent the recombination of electron–hole pairs. The calculated absorption coefficients indicate an obvious redshift of the absorption edge, which is in good agreement with experimental results. This work provides an important indication of how to promote the photocatalytic activity in g-C3N4(001)/BiVO4(010) nanocomposites.
Acknowledgements This work was supported by the Program for Innovative Research Team in the University of Henan Province (no. 13IRTSTHN017), the NSF from department of education of Guizhou province (no. 2114118006zx, QJTD[2013]16), the GZNC
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startup package (no. 13BS027), the Construction Project for Guizhou Provincial Key Laboratories (no. ZJ[2011]4007, ZJ[2013]4009), and the Program for Innovative Research Team of Guizhou Province (No. QKTD[2014]4021). M. S. D would also like to acknowledge the support from the Excellent Youth Scientific and Technological Talents of Guizhou Province (no. QKH-RZ[2013]01).
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