Article pubs.acs.org/IC
Electronic Structure and Optical Properties of BiOI Ultrathin Films for Photocatalytic Water Splitting Zong-Yan Zhao* and Wen-Wu Dai Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, People’s Republic of China S Supporting Information *
ABSTRACT: As a promising photocatalyst driven by visible light, BiOI suffers from its lower conduction band edge position, which leads to its inability to produce hydrogen from photocatalytic water splitting. However, BiOI has an open layered intergrowth structure, which makes it easily cleavable along (001) plane. Thus, inspired by the progress of graphene-like two-dimensional nanomaterials, researchers believe that single-layer BiOI presents excellent photocatalytic activity for water splitting. To further explore the relationship between intrinsic properties and photocatalytic performance of BiOI ultrathin film, its electronic structure and optical properties as a function of layer thickness are systematically investigated by using first-principle calculations. The calculated results indicate that the quantum confinement effects can cause the following variations: band gap increasing, band edge position upshifting, and built-in electric field strengthening, which are very favorable for enhancement of photocatalytic performance. Importantly, if the layer thickness is less than 3 nm, the conduction band edge position will be higher than the reduction potential of H+/H2 and thus appropriate for the overall photocatalytic water-splitting reaction. However, layer thickness also caused disadvantageous reduction of sunlight absorption, which is noticed and avoided in practice. function and the minimization of device.10 The most and famous representative of 2D nanomaterial is graphene. Inspired by the innovation of graphene, the 2D functional materials (or graphene-like material) can be derived from a specific type of materials, which have layered structure (i.e., materials are bonded by the strong chemical interactions within layers and are connected by the weak van der Waals interactions between layers). The latter feature leads to them being easily peeled off into ultrathin films. Bismuth oxyhalides (BiOX, X = F, Cl, Br, and I) just belong to this specific type of materials. It is one of the simplest members of the Sillén family,11 and its structure comprises a layer of [Bi2O2]2+ slabs interleaved by double slabs of halogen ions [X]−, forming [−X−Bi−O−O−Bi−X−] layers stacked one above the other by nonbonding van der Waals interaction through the halogen ions along [001] direction.12 Among BiOXs, BiOI displays the best absorption character in the visible-light range and the best photocatalytic activity for the degradation of organic pollutants under visible-light irradiation.13,14 Furthermore, many types of BiOI with nanostructure have been synthesized and characterized.15,16 However, BiOI has a fatal inborn deficiency: it cannot produce hydrogen from photocatalytic water-splitting reaction or achieve overall photocatalytic water-splitting reaction, because its conduction band (CB) edge position (ca. −5 eV) is lower than the reduction potential of H+/H2 (−4.44 eV at pH = 0).17,18 In recent years, intensive attempts have been dedicated
1. INTRODUCTION Since 1990s, photocatalysis technology is increasingly attracting more and more concerns, because it can convert solar energy into chemical energy to solve the issues of energy shortage and environmental pollution: producing hydrogen by photocatalytic water splitting, converting CO2 to hydrocarbon fuels, or decomposing organic pollutant by photocatalytic degradation reactions.1−3 However, the development of photocatalysis technology suffers from narrow spectral response range for sunlight and low quantum conversion efficiency for solar energy. Recently, researchers are concentrating on the effort of the following two aspects: development of novel photocatalyst or photocatalytic system4,5 and modification of traditional photocatalyst.6,7 With the rapid development of nanotechnology, the development of novel photocatalyst with special nanostructures is becoming one of the most important subjects.8 Nowadays, nanostructure has grown to be an important research subject in the field of photocatalysis technology, which could provide huge specific surface area, rich surface states, and particular device forms. In this field, two-dimensional (2D) nanomaterial received extensive attention, because of its important theoretical value and broad application prospects. 2D nanomaterial has atomic- or molecular-scale thickness and infinite plane size, which not only can enhance the intrinsic performance but also can produce some new properties that do not exist in the corresponding bulk materials.9 In addition, 2D nanomaterial links material microstructure with device performance, so it can really achieve the maximization of the material © XXXX American Chemical Society
Received: July 28, 2015
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DOI: 10.1021/acs.inorgchem.5b01714 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
3. RESULTS AND DISCUSSION Owing to the weak interaction of van der Waals between [−I− Bi−O−O−Bi−I−] layers, to cleave BiOI along (001) plane is very easy. Thus, the cleavage energy or surface energy is very small, as shown in Figure 1. In the case of single-layer BiOI
to improve the photocatalytic activity of BiOI photocatalyst, and some strategies, such as material structural designing,19,20 noble metal decorating,21−23 elemental doping,24 and heterostructure constructing,25−27 have been reported. Ye et al. prepared BiOI thin film that was composed of highly symmetric BiOI nanosheets with dominant exposed (001) facets and found the film has better photocatalytic activity, durability, and selectivity.28 Zhang et al. studied the stabilities and electronic structures of single-layer BiOI by employing first-principles calculations and found this BiOI presents excellent photocatalytic activity for water splitting.29 Although 2D BiOI-based nanomaterials show surprising properties and attractive application foreground, so far, the relevant research has been focused on the synthesis methods and applications. Thus, understanding of the related intrinsic features or the structureperformance relationship is expected to be very scarce. Motivated by the recent experimental and theoretical progress in this subject, we performed first-principles calculations and theoretical analysis to explore the relationship between electronic structure and photocatalytic performance of BiOI ultrathin films. In this article, we present the nature and tendencies of BiOI ultrathin film properties as a function of layer thickness and propose the suitable strategy to enhance its photocatalytic performance for overall water splitting.
Figure 1. Surface energy as a function of layer thickness of BiOI ultrathin film.
2. COMPUTATIONAL METHOD In the present work, the ultrathin film was simulated by using the slab supercell models in which a finite number of layers of BiOI in a threedimensional (3D) periodic supercell were utilized to simulate twodimensional films. For the (001) surface, we chose the (001)-1I stoichiometric terminations as the crystalline facet of BiOI ultrathin films, which is the most stable and has lowest cleavage energy.29,30 When the (001) slab of BiOI was cleaved from the bulk phase, it was separated by a vacuum space of 20 Å from its periodic image along the normal direction. For these models, the layer numbers could represent the thickness of ultrathin film. And then, all of the calculations were performed by using the periodic density functional theory package of Cambridge Serial Total Energy Package codes.31 The core electrons (Bi: [Xe]4f145d10, O: [He], I: [Kr]4d10) were treated with the ultrasoft pseudopotential. The exchange-correlation effects of valence electrons (Bi: 6s26p3, O: 2s22p4, I: 5s25p5) were described by the revised Perdew−Burke−Ernzerhof for solid of generalized gradient approximation (GGA).32 To obtain accurate electronic structure, the method of GGA+U was adopted to overcome the well-known shortcoming of GGA.33 In our previous work, this method has been proved to be useful.17 Furthermore, the DFT-D of van der Waals dispersion corrections was applied to accurately describe the nonbonding van der Waals interaction along c-axis in BiOI.34 The Monkhorst−Pack scheme k-points grid sampling was set as 4 × 4 × 1 for the irreducible Brillouin zone. A 27 × 27 × 180−27 × 27 × 1080 mesh was used for fast Fourier transformation. An energy cutoff of 380 eV was used for expanding the Kohn−Sham wave functions. The minimization algorithm was chosen the Broyden−Fletcher− Goldfarb−Shanno scheme.35 Its convergence criteria were set as follows: the force on the atoms was less than 0.01 eV/Å, the stress on the atoms was less than 0.02 GPa, the atomic displacement was less than 5 × 10−4 Å, and the energy change per atom was less than 1 × 10−6 eV. On the basis of the optimized crystal structure, the electronic structure and the optical properties were then calculated. The band structures were calculated along the paths connecting the following high-symmetry points: Z(0,0,0.5) → A(0.5,0.5,0.5) → M(0.5,0.5,0) → Γ(0,0,0) → Z(0,0,0.5) → R(0,0.5,0.5) → X(0,0.5,0) → Γ(0,0,0) in the k-space.
film, the surface energy is the largest (only 0.012 J/m2). When the layer number is larger than 11 (or the layer thickness is larger than 10 nm), the surface energy converges to 6 × 10−4 J/ m2. The low surface energy indicates that the BiOI ultrathin films or nanosheet can feasibly be prepared by cleaving from the 3D materials via mechanical exfoliation or chemical synthesis, suggesting the preparation cost will be cheap. Along with the decrease of layer thickness, the surface energy is gradually increasing. To clear illustrate this tendency, the calculated data are fitted, indicating that the surface energy presents the rule of exponential decay along with the increasing layer thickness (trending to ∼8.7 × 10−4 J/m2). At the same time, the low surface energy implies another thing: insignificant surface relaxation. Even for single-layer BiOI film, the bond lengths are 2.3308 Å (Bi−O) and 3.3535 Å (Bi−I), which is nearly equal to those in bulk phase (Bi−O: 2.3309 Å; Bi−I: 3.3513 Å). Moreover, in the case of 16-layer BiOI film, the structural parameters of central four layers are approximately equal to those of bulk BiOI. For nanomaterials, the most important phenomenon is the quantum confinement effect: once the size of a material is of the same magnitude as the de Broglie wavelength of the electron wave function, the electronic and optical properties deviate substantially from those of bulk materials. As shown in Figure 2, four calculated band structures along the normal direction of surface (k-point line: Γ → Z) are illustrated for BiOI ultrathin films with 1-(5-, 16-)layers and bulk BiOI. In the case of single-layer BiOI film, the energy eigenvalues are obviously discontinuous. When the layer thickness increases to ∼5 nm, the energy level splitting leads to the increase of energy level number and decrease of energy difference. Furthermore, when the layer thickness is ∼15 nm, the difference between energy levels is further decreasing and forming quasicontinuous energy band. In the case of bulk BiOI, the continuous energy bands are eventually formed, resulting B
DOI: 10.1021/acs.inorgchem.5b01714 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 2. Calculated band structure of 1-(5-, and 16-)layer BiOI film and bulk BiOI along the k-point line of Γ(0,0,0) → Z(0,0,0.5); the minimum band gaps are also presented.
Figure 3. Calculated band gap of BiOI ultrathin films along the normal direction of surface (a) and the lateral direction of surface (b) as a function of layer thickness.
mobility (μ) of single-layer BiOI film, according to the deformation potential theory.36 At the room temperature (T = 300 K), the electron (and hole) mobility of single-layer BiOI film along the lateral direction are 771.19 (and 30.04) cm2 V−1 s−1, and the corresponding relaxation time (τ) are 77.99 (and 16.27) fs, respectively. Therefore, the high carrier mobility and large relaxation time make BiOI ultrathin film rather desirable for photocatalysis. In other words, utilizing the energy quantization effect on the normal direction of BiOI ultrathin film, the photogenerated electrons or holes could fast transfer to the reaction sites on the surface and thus reduce the recombination rate. To further analyze the quantum confinement effect, we distinguish two types of band gaps for BiOI ultrathin film: the minimum band gap along the normal direction of surface (Eg,N) and the minimum band gap along the lateral direction of
from meeting the 3D periodic boundary condition. The aforementioned tendency in variation exactly reflects the feature of 2D electron gas (2DEG): the electronic movement along the normal direction of surface is quantized, and the corresponding energy eigenvalues become discontinuous, while the electronic movement along the lateral direction of surface remains the feature of quasi-free electrons, because the corresponding energy eigenvalues still take continuous values and obey the parabolic variation at the extreme points at the top of valence band (VB) or the bottom of CB (See Figure S1 in Supporting Information). Because of the spatial separation and quantization effect of ultrathin film, it can effectively reduce the scattering effect of the ionized impurities in the carrier transmission process, so the mobility of 2DEG could be greatly enhanced. To better estimate the potentials of BiOI ultrathin films for photocatalysis, we estimated the electron/hole carrier C
DOI: 10.1021/acs.inorgchem.5b01714 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 4. Calculated band edge position (a) and estimated total built-in electric field (b) of BiOI ultrathin film as a function of layer thickness.
Figure 5. Calculated static dielectric constant (a) and refractive index (b) of BiOI ultrathin film as a function of layer thickness; the subscript “P” represents the optical properties are calculated by polycrystalline model.
surface (Eg,L), and their variation with layer thickness is shown in Figure 3 (the corresponding results calculated by GGA method are shown in Figure S2 in Supporting Information). The value of Eg,N exhibits obvious exponential growth along with decreasing layer thickness. The Eg,N of single-layer BiOI film is larger than that of bulk BiOI by ∼0.14 eV. When the layer thickness is larger than 6 nm, the value of Eg,N is gradually converging to ∼2.3 eV. The variation of Eg,L does not exhibit certain tendency, and its difference from that of bulk phase is very gentle (
