Study of the Structure, Electronic and Optical Properties of BiOI/Rutile-TiO2 Heterojunction by the First-Principle Calculation

Using the first-principle calculation that is based on the density functional theory (DFT), our group gains some insights of the structural, electronic and optical properties of two brand new types of BiOI/TiO2 heterojunctions: 1I-terminated BiOI {001} surface/TiO2 (1I-BiOI/TiO2) and BiO-terminated BiOI {001} surface/TiO2 (BiO-BiOI/TiO2). The calculation illustrates that BiOI/TiO2 heterojunction has excellent mechanical stability, and it shows that there is a great possibility for the BiOI/TiO2 heterojunction to be used in visible-light range, hence the photocatalytic ability can be enhanced dramatically. Especially, from the calculation, we discovered that there are two specific properties: the band-gap of 1I-BiOI/TiO2 heterojunction reduces to 0.28 eV, and the BiO-BiOI/TiO2 semiconductor material changes to n-type. The calculated band offset (BOs) for 1I-BiOI/TiO2 heterojunction indicates that the interfacial structure contributes a lot to a suitable band alignment which can disperse the photo-generated carriers into the opposite sides of the interface, so this could effectively weaken the electron-hole recombination. Meanwhile, the built-in potential around the interface accelerates the movement of the photo-generated electron-hole pairs. We believe this is the reason that the BiOI/TiO2 material shows perfect photocatalytic performance. This paper can provide theoretical support for the related research, especially the further research of the BiOI-based material.


Introduction
In modern environmental engineering, photocatalyst has been widely used for activities such as pollution degradation. Titania (TiO 2 ) is a typical candidate, and it has received much attention. Besides environmental engineering, it has been used in many fields like photovoltaics. TiO 2 has perfect properties, such as non-toxic, low-cost and great chemical stability [1][2][3]. But for the energy band properties, TiO 2 is a wide band-gap oxide-semiconductor (3.0 eV for rutile, 3.2 eV for anatase), shows photocatalytic activity under the ultra-violet part of the solar spectrum (λ < 384 nm). This accounts for only a small portion (5%) of the solar energy. This is the reason that the wide bandgap greatly limits its performance of photocatalytic-degradation [4]. Besides, the rapid recombination of photo-induced electron-hole pairs greatly reduces the quantum efficiency of the system and making its photo-generated quantum-yield to be very low [5][6][7][8][9]. Therefore, it is essential for the researchers to develop effective methods of narrowing the band-gap to the visible-light range and improving the charge-separation efficiency of TiO 2 . Therefore, it is highly possible that TiO 2 could be used as an excellent photocatalyst for environmental engineering.
Forming two different semiconductor layers into a heterostructure is an effective method to improve its properties. Under suitable energy band arrangement, the interface could transfer the carriers from a lower energy band to a higher energy band. Meanwhile, the heterojunction helps the separation of photo-induced electron-hole pairs and prevents the recombination of electron-hole pairs, which could greatly enhance the photocatalytic efficiency of semiconductor heterostructure. In previous researches, scientists found that constructing an interface structure of TiO 2 with a narrow-gap material, such as CdS [10,11], CdSe [12,13], CdTe [10], and Cu 2 O [14,15] was considered as a possible method. Under visible-light irradiation, the electron excited from the narrower bandgap semiconductor then transfers to the conduction band of TiO 2 , which can intensify the carrier's transfer and enhances the photocatalytic ability significantly [15][16][17].
Zhang et al. applied a new approach to compose the bismuth oxyhalide BiOX (X = Cl, Br, I) nanospheres [18]. The BiOX with a special layer structure exhibits excellent catalytic properties of degrading pollutants under visible light irradiation. Among those BiOX structures, BiOI is a kind of typical p-type semiconductor with a narrow bandgap of 1.85 eV. It has drawn a lot of attention due to its excellent mechanical and catalytic properties [19][20][21][22]. However, its application is affected by the high carrier-recombination probability. This is a severe bottleneck for the use of BiOI. Due to all these reasons, we hypothesize that the combination of BiOI and TiO 2 into a heterojunction might be a wonderful method to overcome the different disadvantages of the two kinds of semiconductors.
In previous experiments, BiOI/TiO 2 heterojunction has been synthesized and demonstrated excellent photocatalytic performance. It is shown that the performance is better than single BiOI or single TiO 2 [4,23]. However, there is limited theoretical research on BiOI/TiO 2 heterojunction. Especially on the aspect of structural stability, electronic structure, and photocatalytic properties. Due to the absence of relevant theoretical modeling research, it is difficult to further the research of the structure and the photo-catalytic ability. Therefore, we make a theoretical study based on the first-principle method to explore the photocatalytic mechanism of BiOI/TiO 2 heterojunctions in this paper.

Computation Detail and Models
This research is conducted using Materials Studio software (CASTEP) based on the DFT method. Perdew-Burke-Emerhof PBE function of the generalized gradient approximation (GGA) method is used to process the exchange correlation energy [24,25]. A plane-wave cutoff energy (Ecutoff) of 410 eV and ultra-soft pseudo-potential are adopted in the reciprocal K space. A 6 × 6 × 1 Monkhorst-Pack [26] mesh grid is found to be enough to reach convergence for bulk supercell calculations. All the atoms of the heterojunction are fully relaxed to their equilibrium positions with an energy convergence of 5 × 10 −6 eV and the atomic displacement is less than 5 × 10 −4 Å. The calculated parameters have been tested to be credible to get a convergent result in the work. The pseudo-potentials used for the heterojunction are constructed by the electron configurations as Bi-6s6p, O-2s2p, I-5s5p, and Ti-3d4s for the ground-state of electronic structure calculations [27].

The Geometric Structures and Formation Energy of BiOI/TiO2 Heterojunction
The theoretically optimized model of both 1I-BiOI/TiO2 and BiO-BiOI/TiO2 heterojunction is shown in Figure 1c

The Geometric Structures and Formation Energy of BiOI/TiO 2 Heterojunction
The theoretically optimized model of both 1I-BiOI/TiO 2 and BiO-BiOI/TiO 2 heterojunction is shown in Figure 1c For the BiO-BiOI/TiO 2 heterojunction, as shown in Table 1, the Mulliken's bonding population of O (L-2)-Bi (L-4) bond is 0.08 eV, indicating that Bi-O bonds at the interface have a large part of ionic properties and little covalent properties. Another bond population of Bi-O and Ti-O bonds in the BiOI/TiO 2 interface is zero, which means that ionic property dominates in those bonds.

Bond Distance (Å) Bond Population (e) Before Relaxation
After Relaxation To contrast the structural stability of those two BiOI/TiO 2 heterojunctions, we calculate the formation energy (E form ) using the following formula [32]: E total and E BiOI represent the calculated DFT energy for the 1I or BiO terminated BiOI {001} surface model; n Ti and n O represent the numbers of Ti and O atom in TiO 2 slab; µ Ti and µ O are the chemical-potential of the Ti and O atoms; A represents the cross-section area of the interface for the BiOI/TiO 2 heterojunction; µ Ti represents the Ti atomic energy in the bulk Ti; µ O represents half of the total energy of oxygen molecules. The calculated formation energy E form of 1I/TiO 2 and BiO/TiO 2 heterojunctions are −2.21 eV/Å 2 and −2.25 eV/Å 2 , respectively. The E form of the heterojunctions are negative indicating the formations of BiOI/TiO 2 heterojunctions are exothermic and spontaneous, and BiOI and TiO 2 can easily combine. Furthermore, it can be found that BiO-BiOI/TiO 2 is more stable than the 1I-BiOI/TiO 2 heterojunction.

Electronic Structure of BiOI/TiO 2 Heterojunction
The band structures are computed along the special lines connecting the following high-symmetry points: Γ(0,0,0), F(0,0.5,0), Q(0,0.5,0.5), Z(0,0,0.5) and Γ(0,0,0). Figure 2 demonstrates the band structures for the two heterojunctions. It shows that the conduction band maximum (CBM) for the two types of heterojunctions is at Γ point, while the conduction band minimum (VBM) is at Z point. This indicates the typical characteristic of indirect bandgap material [31]. The band-gap of 1I-BiOI/TiO 2 heterojunction is 0.28 eV. This band-gap is decreased a lot compared to the pure BiOI or the TiO 2 .
While for BiO-BiOI/TiO 2 heterojunction, the Fermi level is located at the CBM which means it exhibits the property of n-type semiconductor. A smaller band-gap can produce the redshift phenomenon for 1I-BiOI/TiO 2 heterojunction. it exhibits the property of n-type semiconductor. A smaller band-gap can produce the redshift phenomenon for 1I-BiOI/TiO2 heterojunction. Due to the lattice mismatch, there exist two kinds of strains. The in-plane biaxial tensile strain (ε > 0) and compressive strain (ε < 0). The strain in the {001} plane of the interface between the BiOI and TiO2 is induced by the relative difference of lattice constants. The in-plane biaxial tensile strains (IPBTSs) is calculated by the following formula: where a and 0 a denote the modified and unmodified lattice constants of the 1I-BiOI/TiO2 heterojunction respectively. To understand the influence of strain on the electronic properties of 1I-BiOI/TiO2 heterojunction, we applied a {001} IPBTSs to it. The applied strains vary from −2% to 2% by changing the lattice parameters of 1I-BiOI/TiO2 heterojunction. Figure 3 demonstrates the band structures of the 1I-BiOI/TiO2 heterojunction under different {001} IPBTSs. The band gaps are 0.21 eV, 0.23 eV, 0.30 eV and 0.33 eV for −2%, −1%, 1% and 2% IPBTSs respectively. Notably, the band gaps of those modified materials increase with the increasing strain of the 1I-BiOI/TiO2 heterojunction; meanwhile, the variation of the band-gap changes are all in the range of 0.12 eV. Therefore, the result of the band gap in our calculation is reliable.   Due to the lattice mismatch, there exist two kinds of strains. The in-plane biaxial tensile strain (ε > 0) and compressive strain (ε < 0). The strain in the {001} plane of the interface between the BiOI and TiO 2 is induced by the relative difference of lattice constants. The in-plane biaxial tensile strains (IPBTSs) is calculated by the following formula: where a and a 0 denote the modified and unmodified lattice constants of the 1I-BiOI/TiO 2 heterojunction respectively. To understand the influence of strain on the electronic properties of 1I-BiOI/TiO 2 heterojunction, we applied a {001} IPBTSs to it. The applied strains vary from −2% to 2% by changing the lattice parameters of 1I-BiOI/TiO 2 heterojunction. Figure 3 demonstrates the band structures of the 1I-BiOI/TiO 2 heterojunction under different {001} IPBTSs. The band gaps are 0.21 eV, 0.23 eV, 0.30 eV and 0.33 eV for −2%, −1%, 1% and 2% IPBTSs respectively. Notably, the band gaps of those modified materials increase with the increasing strain of the 1I-BiOI/TiO 2 heterojunction; meanwhile, the variation of the band-gap changes are all in the range of 0.12 eV. Therefore, the result of the band gap in our calculation is reliable. it exhibits the property of n-type semiconductor. A smaller band-gap can produce the redshift phenomenon for 1I-BiOI/TiO2 heterojunction. Due to the lattice mismatch, there exist two kinds of strains. The in-plane biaxial tensile strain (ε > 0) and compressive strain (ε < 0). The strain in the {001} plane of the interface between the BiOI and TiO2 is induced by the relative difference of lattice constants. The in-plane biaxial tensile strains (IPBTSs) is calculated by the following formula: where a and 0 a denote the modified and unmodified lattice constants of the 1I-BiOI/TiO2 heterojunction respectively. To understand the influence of strain on the electronic properties of 1I-BiOI/TiO2 heterojunction, we applied a {001} IPBTSs to it. The applied strains vary from −2% to 2% by changing the lattice parameters of 1I-BiOI/TiO2 heterojunction. Figure 3 demonstrates the band structures of the 1I-BiOI/TiO2 heterojunction under different {001} IPBTSs. The band gaps are 0.21 eV, 0.23 eV, 0.30 eV and 0.33 eV for −2%, −1%, 1% and 2% IPBTSs respectively. Notably, the band gaps of those modified materials increase with the increasing strain of the 1I-BiOI/TiO2 heterojunction; meanwhile, the variation of the band-gap changes are all in the range of 0.12 eV. Therefore, the result of the band gap in our calculation is reliable.   The total and partial density of states (DOS) for the two heterojunctions depict in Figure 4. It shows the difference between TDOS and PDOS for those heterojunctions. As we can see, in Figure 4, for those two types of heterojunctions, VB is primarily made up of I 5p, O 2p and little Bi 6p states and the VBM is primarily made up of O 2p states, while the CB is chiefly consisted of Bi 6p, Ti 3d, O 2p states. The CBM primarily consists of Bi 6p and Ti 3d states. It reveals that the reason why the photo response for the 1I-BiOI/TiO 2 heterojunction is that the electron transfers from the O 2p in VB to the hybridized Bi 6p and Ti3d states in the CB. Figure 4b presents the DFT-calculated TDOS and PDOS of BiO-BiOI/TiO 2 heterojunction. It can be seen that the CBM downward movement 1.10 eV contrasts to the 1I-BiOI/TiO 2 heterojunction promoting the Fermi level through the CB and makes it an n-type semiconductor.
Materials 2020, 13, x FOR PEER REVIEW 6 of 13 The total and partial density of states (DOS) for the two heterojunctions depict in Figure 4. It shows the difference between TDOS and PDOS for those heterojunctions. As we can see, in Figure 4, for those two types of heterojunctions, VB is primarily made up of I 5p, O 2p and little Bi 6p states and the VBM is primarily made up of O 2p states, while the CB is chiefly consisted of Bi 6p, Ti 3d, O 2p states. The CBM primarily consists of Bi 6p and Ti 3d states. It reveals that the reason why the photo response for the 1I-BiOI/TiO2 heterojunction is that the electron transfers from the O 2p in VB to the hybridized Bi 6p and Ti3d states in the CB. Figure 4b presents the DFT-calculated TDOS and PDOS of BiO-BiOI/TiO2 heterojunction. It can be seen that the CBM downward movement 1.10 eV contrasts to the 1I-BiOI/TiO2 heterojunction promoting the Fermi level through the CB and makes it an n-type semiconductor.

Band Offset and Charge Transfer
When two kinds of semiconductors combine into a heterojunction, the band structure of the interface in the heterojunction will form a discontinuous condition. The discontinuous condition finally forms band offset (BOs) (i.e., valence band offset (VBO) and conduction band offset (CBO)). BOs is the most important factor to influence the photo-generated carriers' movement through the interface. Two methods can be applied to obtain the BOs; first is the "local density of states (LDOS)" [33,34] and second is the "potential-line-up methods" [28,35,36]. Here, we use the first approach to obtain the Bos. The second approach is also applied to verify the precision of the BOs of the 1I-BiOI/TiO 2 heterojunction.
The DFT method calculated the density of states, including the TDOS and LDOS regarding bilateral layers far from the interface of the 1I-BiOI/TiO 2 heterojunction that is plotted in Figure 5a. The bulk BiOI and bulk TiO 2 , shown in Figure 5a represents the layers far from the interface indicating that the electronic distribution is like that in independent materials. It can be seen from Figure 5a that the valence band and conduction band of bulk BiOI shows a larger shift to the lower energy. In addition, it can be perceived that the band-gap of bulk BiOI is about 1.39 eV, ranging from −0.79 to 0.60 eV, while bulk TiO 2 has a band-gap of 1.91 eV, ranging from 0 to 1.91 eV. The band gap of bulk BiOI and TiO 2 are reduced compared with pure BiOI (1.85 eV) and TiO 2 (3.0 eV). A smaller band-gap of bulk BiOI and TiO 2 will increase the excitation probability of photogenerated electrons, which can stimulate stronger visible light response and enhance the catalytic activity. A substantial charge transfers through the interfaces of 1I-BiOI/TiO2 heterojunction. For making a thorough investigation into the electronic distribution and transfer of the 1I-BiOI/TiO2 heterojunction, the three-dimensional charge density difference (CDD) is used to analyze the As shown in Figure 5b, the valence band maximum (VBM) of the interface is primarily contributed by TiO 2 . The difference VBM between BiOI and TiO 2 is 0.79 eV. Therefore, the VBO between BiOI and TiO 2 is 0.79 eV. Here, the CBO is deduced by utilizing the band gaps of BiOI and TiO 2 from the experiment (3.0 eV for TiO 2 and 1.85 eV for BiOI). The VBO is calculated by the following formula [37][38][39]: ∆E expt gap represents the difference between the experimental optical bands gap of BiOI and TiO 2 . The CBO calculated by this formula is 1.94 eV.
The VBO can be deduced through the "potential-line-up method" by the following formula [37,38]: ∆E V is the difference of the VBM got from the pour structures of BiOI and TiO 2 under the same pressure. ∆V is obtained through the calculation of the difference of the average of the electrostatic potential through the 1i-BiOI/TiO 2 heterojunction.
The calculated electrostatic potentials of 1I-BiOI and TiO 2 are plotted in Figure 5b. The solid red lines represent the average potential of BiOI and TiO 2 parts. The average electrostatic potentials of BiOI and TiO 2 are −11.60 eV and −19.96 eV respectively, indicating that ∆V is −8.36 eV. While the VBM of BiOI and TiO 2 are −3.72 eV and 5.67 eV respectively indicating the calculated ∆E V is 9.39 eV, so the deduced value of VBO is 1.03 eV. The deduced VBO calculated by the potential-line-up method is about 1.03 eV matching well with the value of the LDOS analysis. As shown in Figure 5b, a built-in potential about 28.3 eV is created around the interface of 1I-BiOI/TiO 2 junction. Generally, the calculated VBO and CBO of 1I-BiOI/TiO 2 heterojunction are 0.79 eV and 1.94 eV respectively, which indicates that the BOs between BiOI and TiO 2 will enhance the separation of photogenerated electron-hole pairs and effectively avoid electron-hole recombination. Therefore, electrons could be transferred from the TiO 2 to BiOI section and the holes will be transferred in the opposite direction. In addition, the huge difference of potential through the interface capacity to stimulate separation of electron-hole pairs to the farther ends and finally photocatalytic efficiency can be improved.
A substantial charge transfers through the interfaces of 1I-BiOI/TiO 2 heterojunction. For making a thorough investigation into the electronic distribution and transfer of the 1I-BiOI/TiO 2 heterojunction, the three-dimensional charge density difference (CDD) is used to analyze the properties of 1I-BiOI/TiO 2 heterojunction. CDD can describe the "electronic map" vividly and intuitively. This practical method can be deduced by the following formula: ∆ρ, ρ TiO 2 and ρ BiOI represent the electronic density of the 1I-BiOI/TiO 2 heterojunction, the free-standing isolated TiO 2 and the 1I-BiOI surface in the same configuration, respectively.
From Figure 6a, we found that there is a dramatic change of the electronic density around the interface owing to the combination of TiO 2 and BiOI. Significant electronic accumulation is observed in the space above I (L-3) atoms in 1I-BiOI part, whereas a little charge depletion is found both on the bottom of Ti (L-2) atoms in the lowest side of the TiO 2 part and the middle of the interface region of the 1I-BiOI/TiO 2 heterojunction. The above analysis indicates that the electron transport violently occurs around the interface owing to the combination of BiOI and TiO 2 . The Mulliken population can present the quantitative calculation of the electronic distribution for the 1I-BiOI/TiO2 heterojunction. Here, the Mulliken charge of two representative atoms (I and Ti atoms) is demonstrated in Figure 6b. As shown in Figure 6, the I (L-3) atom obtains a Mulliken charge of −0.30, which is greater than I atoms in other layers of 1I-BiOI part, clearly demonstrating that the electron of I atoms (L-3) increases by a very small amount in the 1I-BiOI/TiO2 heterojunction. The charge variation demonstrates that the I (L-3) atom in the top layer of the BiOI part would lose less electrons than I atoms inside the 1I-BiOI part. While the Mulliken charge of Ti (L-2) is 1.23, which is smaller than Ti atoms in other layers, indicating that the electron redistribution appears around the interface. I (L-3) atoms lose some electrons to Ti (L-4) atoms, indicating that the Mulliken charge analysis is consistent with the CDD analysis. Meanwhile, the built-in potential is created around the interface, and the built-in potential drive electronic transfer from the TiO2 part to 1I-BiOI part, which is consistent with the band offset analysis in Figure 5a.

Optical Properties and Photocatalytic Mechanism
The dielectric function provides a close connection between photons and electrons, and it reflects the linear response of materials to electromagnetic radiation [40]. Absorption spectra can be calculated according to the following formula [41]: The calculated optical absorption spectra of pure BiOI, TiO2 and BiOI/TiO2 heterojunctions systems as the function of energy and wavelength are displayed in Figure 7. It can obtain that at the low-energy region from 0 to 1 eV, as shown in in Figure 7a, the optical absorption spectra of BiO-BiOI/TiO2 heterojunction has a small characteristic peak at 0.69 eV. The optical absorption amount of BiOI/TiO2 heterojunction is much higher than that of pure BiOI and TiO2 in the low-energy region. In addition, composing BiOI and TiO2 into a heterojunction makes the absorption edges redshift, and The Mulliken population can present the quantitative calculation of the electronic distribution for the 1I-BiOI/TiO 2 heterojunction. Here, the Mulliken charge of two representative atoms (I and Ti atoms) is demonstrated in Figure 6b. As shown in Figure 6, the I (L-3) atom obtains a Mulliken charge of −0.30, which is greater than I atoms in other layers of 1I-BiOI part, clearly demonstrating that the electron of I atoms (L-3) increases by a very small amount in the 1I-BiOI/TiO 2 heterojunction. The charge variation demonstrates that the I (L-3) atom in the top layer of the BiOI part would lose less electrons than I atoms inside the 1I-BiOI part. While the Mulliken charge of Ti (L-2) is 1.23, which is smaller than Ti atoms in other layers, indicating that the electron redistribution appears around the interface. I (L-3) atoms lose some electrons to Ti (L-4) atoms, indicating that the Mulliken charge analysis is consistent with the CDD analysis. Meanwhile, the built-in potential is created around the interface, and the built-in potential drive electronic transfer from the TiO 2 part to 1I-BiOI part, which is consistent with the band offset analysis in Figure 5a.

Optical Properties and Photocatalytic Mechanism
The dielectric function ε(ω) = ε 1 (ω) + iε 2 (ω) provides a close connection between photons and electrons, and it reflects the linear response of materials to electromagnetic radiation [40]. Absorption spectra can be calculated according to the following formula [41]: The calculated optical absorption spectra of pure BiOI, TiO 2 and BiOI/TiO 2 heterojunctions systems as the function of energy and wavelength are displayed in Figure 7. It can obtain that at the low-energy region from 0 to 1 eV, as shown in in Figure 7a, the optical absorption spectra of BiO-BiOI/TiO 2 heterojunction has a small characteristic peak at 0.69 eV. The optical absorption amount of BiOI/TiO 2 heterojunction is much higher than that of pure BiOI and TiO 2 in the low-energy region.
In addition, composing BiOI and TiO 2 into a heterojunction makes the absorption edges redshift, and the BiO-BiOI/TiO 2 heterojunction has a larger amount of redshift than 1I-BiOI/TiO 2 heterojunction. The above analysis makes it clear that the BiOI/TiO 2 heterojunction can dramatically enhance the performance of absorbing visible light, and finally improve the photocatalytic performance.
in the VB of p-BiOI part priority. The reformed VB edge of BiOI is more inactive than that of TiO2 about 0.79 eV. Hence photo-induced holes on the VB of BiOI part would easily transfer to the VB of TiO2 part while leaving the photo-induced electrons on the CB of BiOI as shown in Figure 8. Electrons will be driven by transferring from CB of TiO2 to BiOI part driven by built-in potential through the surface at the same time. The formed built-in potential at the interface of 1I-BiOI/TiO2 heterojunction can significantly accelerate the transfer of photogenerated carriers and prevent its recombination dramatically. The existence of those well-separated photo-electrons and holes would better degrade molecules of organic pollutants. On the whole, the BiOI/TiO2 heterojunction possesses excellent photocatalytic properties under the degradation of organic pollutants condition under illumination.   From the above analysis of the electronic structures and optical properties of the 1I-BiOI/TiO 2 heterojunction, a schematic diagram of band structure for pre and post binding 1I-BiOI/TiO 2 heterojunction is shown in Figure 8. We can obtain the conclusions as follows: BiOI with narrow band-gap (1.85 eV) can be tendentiously excited by most of visible light and promote the generation of photoelectrons and holes. However, the generated photo-induced electron-hole pairs of BiOI may recombine immediately due to the narrow band-gap and small sizes, which weakening its photocatalytic properties to a great extent. Because of the large band-gap of TiO 2 (3.0 eV), TiO 2 can only use a few parts of visible-light (energy >2.95 eV, wavelength <420 nm). As shown in the schematic diagram of Figure 8, when BiOI and TiO 2 are contacted with each other constituting a heterojunction, BiOI will play a "photoelectric conversion driver" to absorb visible light. A large built-in potential about 28.3 eV, shown in Figure 5b, and it is created around the interface of the 1I-BiOI/TiO 2 heterojunction. Therefore, electrons could be pumped from the TiO 2 to the BiOI section and the holes will transfer in the opposite direction. Meanwhile, the built-in potential capacity will promote photogenerated electron-hole pairs separation on the contrary direction.

Conclusions
The characteristics and properties of the 1I-BiOI/TiO2 heterojunction and the BiO-BiOI/TiO2 heterojunction are studied via the first-principles calculation based on the DFT theory. The structure of the BiOI/TiO2 changes slightly, and the interaction on the interface is van der Waals rather than covalent. The band structure of the BiOI/TiO2 heterojunction is modified a lot. The band-gap of the 1I-BiOI/TiO2 heterojunction reduces to 0.28 eV and the BiO-BiOI/TiO2 heterojunction changes into an n-type semiconductor. The calculated VBO of the 1I-BiOI/TiO2 heterojunction is 0.79 eV, and the CBO of the 1I-BiOI/TiO2 heterojunction is 1.94 eV respectively. The generated electron-hole pairs will be preferentially produced in the BiOI side under light, and then the holes are easily transferred through the interface from the VBM of the BiOI part to the VBM of the TiO2 part while leaving the electrons in the CBM of BiOI part. Meanwhile, the built-in potential can effectively speed up the separation of photo-generated electron-hole pairs and effectively avoid electron-hole recombination. Perfectly separated electron-hole pairs across the interface of the BiOI/TiO2 heterojunction will greatly improve its photocatalytic performance. The conclusions in this paper can provide a As a result, under the light condition (energy >1.85 eV), photogenic positive holes are stimulated in the VB of p-BiOI part priority. The reformed VB edge of BiOI is more inactive than that of TiO 2 about 0.79 eV. Hence photo-induced holes on the VB of BiOI part would easily transfer to the VB of TiO 2 part while leaving the photo-induced electrons on the CB of BiOI as shown in Figure 8. Electrons will be driven by transferring from CB of TiO 2 to BiOI part driven by built-in potential through the surface at the same time. The formed built-in potential at the interface of 1I-BiOI/TiO 2 heterojunction can significantly accelerate the transfer of photogenerated carriers and prevent its recombination dramatically. The existence of those well-separated photo-electrons and holes would better degrade molecules of organic pollutants. On the whole, the BiOI/TiO 2 heterojunction possesses excellent photocatalytic properties under the degradation of organic pollutants condition under illumination.

Conclusions
The characteristics and properties of the 1I-BiOI/TiO 2 heterojunction and the BiO-BiOI/TiO 2 heterojunction are studied via the first-principles calculation based on the DFT theory. The structure of the BiOI/TiO 2 changes slightly, and the interaction on the interface is van der Waals rather than covalent. The band structure of the BiOI/TiO 2 heterojunction is modified a lot. The band-gap of the 1I-BiOI/TiO 2 heterojunction reduces to 0.28 eV and the BiO-BiOI/TiO 2 heterojunction changes into an n-type semiconductor. The calculated VBO of the 1I-BiOI/TiO 2 heterojunction is 0.79 eV, and the CBO of the 1I-BiOI/TiO 2 heterojunction is 1.94 eV respectively. The generated electron-hole pairs will be preferentially produced in the BiOI side under light, and then the holes are easily transferred through the interface from the VBM of the BiOI part to the VBM of the TiO 2 part while leaving the electrons in the CBM of BiOI part. Meanwhile, the built-in potential can effectively speed up the separation of photo-generated electron-hole pairs and effectively avoid electron-hole recombination. Perfectly separated electron-hole pairs across the interface of the BiOI/TiO 2 heterojunction will greatly improve its photocatalytic performance. The conclusions in this paper can provide a theoretical understanding of the BiOI/TiO 2 heterojunction and the further utilization of visible-light response materials, such as BiOI-based semiconductor photocatalysts.