Letter pubs.acs.org/JPCL
Visible-Light-Absorption in Graphitic C3N4 Bilayer: Enhanced by Interlayer Coupling Fang Wu,† Yunfei Liu,‡ Guanxia Yu,† Dingfeng Shen,† Yunlu Wang,§ and Erjun Kan*,∥ †
School of Science, Nanjing Forestry University, Nanjing, Jiangsu 210037, P. R. China College of Information Science and Technology, Nanjing Forestry University, Nanjing, Jiangsu 210037, P. R. China § School of Wood Science and Technology, Nanjing Forestry University, Nanjing, Jiangsu 210037, P. R. China ∥ Department of Applied Physics, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, P. R. China ‡
S Supporting Information *
ABSTRACT: Although graphitic C3N4 (g-C3N4) has been demonstrated to be a potential candidate for solar cell absorber and photovoltaic materials, the application has been limited by the low photoconversion efficiency in the visible range. Here, we explored that a g-C3N4 bilayer has much better visible-light adsorption than a single layer via first-principles calculations, and the calculated optical adsorption threshold of bilayer significantly shifts downward by 0.8 eV, which is induced by the interlayer coupling. Additionally, we also found that the optical energy gap of bilayer can be engineered by the external electric field. The insights obtained in this study are general and will be helpful for future studies of twodimensional solar cell absorber and photovoltaic materials. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis
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not much research studying the efficiency of visible-light adsorption. Thus, the interesting question is how to make gC3N4 a suitable photovoltaic material/solar cell absorber by enhancing the visible-light adsorption. In this paper, by performing first-principles calculations, we show that a gC3N4 single layer has a moderate direct energy gap (1.25 eV), and the weak visible-light absorption comes from the negligible orbital overlap. For a g-C3N4 bilayer, the interlayer coupling increases the fundamental band gap (1.9 eV). Remarkably, we found the visible-light adsorption of a bilayer has been significantly enhanced by shifting the optical adsorption threshold downward. Additionally, we also found that the optical energy gap of a bilayer can be easily engineered by the external electric field. The atomic understanding obtained in our study can be applied for future studies of other twodimensional solar cell absorber/photovoltaic materials. Calculation Methods. Our first-principles calculations were based on density functional theory (DFT) using the generalized gradient approximation (GGA) known as PW91,18 implemented in the Vienna ab initio simulation package (VASP) code.19 The projected augmented wave (PAW) method20,21 with a plane-wave basis set was used. For the spin-polarized calculations, the Vosko−Wilk−Nusair modification22 scheme was applied to interpolate the correlation energy. We applied periodic boundary conditions with a vacuum space of 20 Å in order to avoid interactions between bilayers in nearest-neighbor unit cells. All of the structures were relaxed
ue to renewable energy and clean environment, significantly efforts have been devoted to search new solar cell absorber and photovoltaic materials, which can convert solar energy to chemical energy.1−3 In terms of energy conversion and commercial applications, the efficiency of the visible-light adsorption/activity becomes the determining factor in the field of solar cell absorber and photovoltaic materials, because near half of the solar energy comes from the visiblelight part. In recent years, several groups have successfully reported visible-light-active photovoltaic materials/solar cell absorbers.4,5 For example, Watkins et al. reported growing a new monocrystalline AlPSi3 phase on Si substrates via tailored interactions,5 and the following first-principles calculations demonstrate6 that AlPSi3 has high absorption in the visible-light region, indicating a new promising material for solar cell absorbers. Compared with traditional bulk materials, two-dimensional atomic crystals have an intrinsic advantage as photovoltaic materials/solar cell absorbers because of the limited thickness and large surface. However, the inadequate band gap of most two-dimensional atomic crystals has closed the access to photovoltaic materials/solar cell absorbers.7−14 Recently, graphitic C3N4 (g-C3N4) has been explored as a promising candidate for solar energy conversion.15 However, the reported results show that pure g-C3N4 adsorbs only blue light up to 450 nm, which is unfavorable for photovoltaic devices or solar cell absorbers. To get better performance in photovoltaic materials/solar cell absorbers, it is necessary to enhance the visible-light absorption of g-C3N4. Although several methods have been proposed to modify the photocatalytic properties,16,17 there is © 2012 American Chemical Society
Received: September 28, 2012 Accepted: October 26, 2012 Published: October 26, 2012 3330
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and the N3 atoms of the top layer occupy holes of the bottom layer. The calculated binding energy of such a bilayer is 1.21 eV, reflecting strong chemical bonding. It is totally different with the weak interactions in other two-dimensional bilayers, such as a graphene bilayer,23 and a graphene/BN bilayer.24 Additionally, we also found that there are several metastable structures, and their relative energy is calculated as shown in Figure 2. (Please see the details of the optimized GS and MS structures in the Supporting Information.) To explore the bilayer stacking effect on the electronic properties, we calculated the band dispersions of GS and three MS structures. As shown in Figure 3a, all four structures show the semiconducting character with a direct energy gap around 1.9 eV. Compared with a g-C3N4 single layer, we found that there are two significant characters: (1) the band dispersions have switched from indirect character into a direct one. (2) The fundamental band gap of the bilayer has been enlarged by roughly 0.65 eV. To understand the calculated electronic properties, we plotted the charge difference (ρdiff) of the GS structure, which is defined as ρdiff = ρby − (ρst + ρsb), where ρby is the charge density of the bilayer, and ρst and ρsb are the isolated charge densities of the top and bottom layers. As shown in Figure 3b, by forming a bilayer structure, the N3−C bonding in one single layer is significantly reduced, and N2 atoms of the top layer form covalent bonds with the N2 atoms of the bottom layer. The picture obtained by the charge difference plot is also supported by PDOS analysis. The states around the VBM of the N2 atoms are significantly broadened, which indicates N2−N2 bonding. Additionally, we also found that the first peak of the occupied states of carbon and N1 atoms has been upward shifted to the VBM. By forming bilayer structrures, the nonbonding character of the N2 atoms is saturated, leading to the reduced energy split between VBM and N1/C atoms. Optical Properties. Since most of the solar energy comes from the visible-light region, it is naturally expected that photovoltaic materials/solar cell absorbers can absorb visiblelight to enhance their efficiency. According to our electronic structure calculations, a g-C3N4 single layer has a moderate direct energy gap, which should be a suitable visible-light absorber. However, the reported experimental results have demonstrated that a g-C3N4 single layer has only weak visiblelight-activity. Thus, it is important to explore the relationship of optical properties and electronic structures. We first calculated the optical properties of a g-C3N4 single layer. Figure 4a shows the calculated imaginary dielectric functions of a single layer using GGA. We found that a g-C3N4 single layer only has adsorption above 2 eV, which is much larger than the minimum direct energy gap at the M point (Figure 1b). Figure 4b shows the plotted charge density of conductive band minimum (CBM) and VBM at the M point. It is clear that the orbital overlap and hence the optical transitions between CBM and VBM are negligible, leading to lightabsorption in a high energy region. To enhance the optical absorption in the visible-light regions, it is necessary to mix different orbitals of a single layer to fulfill the optical selection rules. Since bilayer structures have interlayer coupling, they may have better optical properties. Figure 4a shows the calculated imaginary dielectric functions of GS and three MS structures. All four structures have similar optical behaviors, and have the much better optical absorption below 2 eV. This can be understood by the fact that the interlayer coupling has modified the orbital, leading to stronger
using the conjugated gradient method without any symmetric constraints. We set the energy cutoff and convergence criteria for energy and force to be 550 eV, 10−4 eV, and 0.01 eV/Å, respectively. During the optimization, 11 × 11 × 1 K-point is adopted, while 15 × 15 × 1 is used for total energy calculations. For the optical spectrum calculations, we used the DFT-GGA code to calculate the imaginary dielectric functions for qualitative comparisons, and all the results have been checked by 21 × 21 × 1 kpoints. Electronic Structures. For a g-C3N4 single layer, due to the different chemical environments, nitrogen atoms are separated into three kinds, namely, N1, N2, and N3, while all carbon atoms have three nearest-neighbor nitrogen atoms, as plotted in Figure 1a. Carefully looking at the geometric structures, we
Figure 1. (a) Geometric structures of a g-C3N4 single layer: gray balls are nitrogen atoms, and yellow ones are carbon atoms. The blue lines represent the unit cell adopted in our calculations. The calculated (b) band dispersion and (c) projected density of states of a g-C3N4 single layer.
found that both N1 and N3 atoms are fully saturated by the surrounding carbon atoms, while N2 atoms only connect two carbon atoms, leaving a nonbonding character. By performing DFT-GGA calculations, we found that a gC3N4 single layer is a semiconductor with an indirect energy gap of 1.19 eV (Figure 1b). The minimum direct energy gap is about 1.25 eV. To obtain more information about the electronic structures of a g-C3N4 single layer, we also plotted the partial density of states (PDOS). As shown in Figure 1c, the electrons of N1, N3, and carbon atoms only occupy the states far away from the valence band maximum (VBM), and the VBM is dominated by the N2 atoms. Thus, the calculated PDOS well reflects the different chemical bonding environment as shown in the geometric structures. To study the electronic and optical properties of a g-C3N4 bilayer, we constructed 16 kinds of nonequivalent structures according to symmetry. For the most stable structure of g-C3N4 bilayer, we found that N1 and N2 atoms of the top layer almost located above the relative N1 and N2 atoms of the bottom one, 3331
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Figure 2. Optimized structures of a g-C3N4 bilayer, where GS, MS1, MS2, and MS3 mean the ground structure and three metastable structures. The bottom layer is plotted with balls, and the top layer is represented with sticks. The calculated relative energy of different structures is also shown in the red box.
Figure 3. (a) The calculated band dispersions of GS and three MS structures. (b) Charge difference plots of GS bilayer, which is defined as ρdiff = ρby − (ρst + ρsb), where ρby is the charge density of a bilayer, and ρst and ρsb are the isolated charge densities of the top and bottom layer. The top one is the top view, and the bottom one is the side view. Red color shows the charge accumulated regions, and blue color shows the charge depleted regions. (c) Plotted PDOS of the GS bilayer.
overlap of CBM and VBM of a bilayer at the M point, as shown in Figure 4c. The interlayer coupling between the bilayer plays a crucial role in the optical properties. Moreover, although general DFT always underestimates the band gap, it has been shown that the tendency of calculated optical properties is reasonable.6 Thus, our predictions give a practical way to enhance the visible-light-absorption of g-C3N4. Band Engineering. Since the optical gap of a GS bilayer is determined by the fundamental band gap, it is interesting to answer whether it can be tuned by the external electric field, as reported in other similar systems.8 As the diagram plotted in Figure 5, we use a perpendicular electric field to see the effect
on the electronic structures of GS bilayer. The calculated band gap shows linear reductions with increasing the external electric field. By plotting the band dispersion, we found that the main character of the GS bilayer is not changed by the electric field. Similar to the pristine GS bilayer, both the CBM and VBM are located at the K point. Thus, our results show that the electronic structures and hence the optical properties of a GS bilayer can be further controlled by an external electric field. In summary, using DFT calculations, we have systematically studied the electronic and optical properties of a g-C3N4 single layer and bilayer. Our results reveal that although a g-C3N4 single layer has a moderate energy gap, the optical absorption 3332
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Letter
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (11204137, 21203096), by the Natural Science Foundation of Jiangsu Province (BK2012392), by the Jiangsu Province Universities Natural Science Foundation (11KJB140002), and by NJUST Research Funding (No. 2011ZDJH02, AB41374, and AE88069). We also acknowledge the support from the Shanghai Supercomputer Center.
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Figure 4. (a) Calculated imaginary dielectric functions versus energy within the GGA framework for a g-C3N4 single layer and bilayer. (b,c) Charge density of the CBM and VBM at the M point for a single layer and GS bilayer, respectively.
Figure 5. The calculated energy gap of a GS bilayer as a function of external electric filed. The left inset figure is the band structure of a GS bilayer as the electric field of 0.4 V/Å, and the right inset figure is the diagram of a GS bilayer with an electric field. The positive direction of the electric field is denoted by the arrow.
in the visible-light regions is negligible. For a g-C3N4 bilayer, the fundamental band gap is increased by the interlayer coupling, and the calculated optical absorption shows that it has much better visible-light absorption. Additionally, we also show that the energy of bilayer can be easily tuned by external electric field, which may be used to control its optical absorption. In view of the most recent progress on low-dimensional photovoltaic materials, our results pave a practical way to achieve two-dimensional solar cell absorber/photovoltaic materials.
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REFERENCES
ASSOCIATED CONTENT
S Supporting Information *
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