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First-principles study of electronic structure and optical properties of (Zr–Al)-codoped ZnO
Computational Materials Science 82 (2014) 70–75
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First-principles study of electronic structure and optical properties of (Zr–Al)-codoped ZnO Jin-Hua Luo, Qun Liu, Li-Na Yang, Zhu-Zhu Sun, Ze-Sheng Li ⇑ Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry, Beijing Institute of Technology, Beijing 100081, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 3 July 2013 Received in revised form 5 September 2013 Accepted 6 September 2013 Available online 15 October 2013 Keywords: First principles (Zr–Al)-codoped ZnO Optical absorption Oxygen vacancy Zinc vacancy
a b s t r a c t The Heyd–Scuseria–Ernzerhof (HSE) hybrid density functional was used to investigate the electronic structure and optical properties of (Zr–Al)-codoped ZnO. The calculated results show that the formation energy of (Zr–Al)-codoped ZnO is low, indicating that it is the energetically favorable structure and the first absorption peak of optical absorption spectra for (Zr–Al)-codoped ZnO has a red-shift compared with pure ZnO, which may lead to the improvement on the visible-light photocatalytic ability. The zinc and oxygen vacancies introduced by (Zr–Al)-codoped ZnO have also been investigated. Through analysis the main absorption peaks of the imaginary part of the dielectric function on polarization vectors perpendicular or parallel to the Z axis, we think that oxygen vacancy introduced by (Zr–Al) codoping can also improve the visible-light photocatalytic ability and zinc vacancy has weak effect to enhance the optical photocatalytic ability compared with (Zr–Al)-codoped ZnO. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction ZnO is a unique semiconductor material with a direct wide band gap (3.37 eV) and a large exciton binding energy (60 meV), which ensures efficient ultraviolet emission [1,2]. As a result, ZnO has been widely used in photoelectric applications, such as transparent conductive oxides (TCOs) [3,4], transistors [5], diluted magnetic semiconductors [6–9], and dye-sensitized solar cells (DSSCs) [10]. In order to design novel semiconductor materials, the theoretical efforts have been devoted to study the electronic and optical properties of ZnO involved materials [1,11–13]. Doping is one of the most efficient processes to improve the structures and properties of this kind of materials [14–16]. The doping of transition metal induces drastic changes in optical, electrical, and magnetic properties of ZnO by altering its electronic structure. Khan et al. [17] demonstrated that the optical absorption spectra for Zr doped ZnO nanoparticles show the absorption in the visible region. Meanwhile, many studies indicated that Al doped ZnO is one of most hopeful candidates for replacing Tin-doped indium oxide, due to its relatively high abundance, low fabrication cost, low toxicity and thermal stability [18–21]. In addition, Al doped ZnO nanorod arrays have been found to improve the energy conversion efficiency of dye-sensitized solar cells significantly [22,23]. Recently, co-doped ZnO have received extensive studies and some research groups have demonstrated that co-doping can modify the electronic struc⇑ Corresponding author. Address: 5 South Zhongguancun Street, Haidian Zone, Beijing 10081, People’s Republic of China. Tel./fax: +86 10 68918670. E-mail address: [email protected] (Z.-S. Li). 0927-0256/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.commatsci.2013.09.021
ture and optical properties of the intrinsic ZnO efficiently [24–26]. Experimentally, Yu et al. reported that (Zr–Al)-codoped ZnO has a greater improvement on the photocatalytic activity [27]. However, compared with the widely studied donor–acceptor codoping in ZnO systems [26,28], the report on the theoretical researches about (Zr–Al)-codoped ZnO system has not been found up to date. Thus, in order to understand the relationship between the photocatalytic activity and the electronic structure and optical properties, theoretical investigation of (Zr–Al)-codoped ZnO system is indispensable. Besides, point defects have an important effect on the electronic and optoelectronic properties of semiconductor materials [29]. The experimental study by Zheng et al. has indicated that the optical and photocatalytic properties of ZnO nanocrystals are dependent on the type and concentration of oxygen defects in the synthesized ZnO nanocrystals [30]. Using theoretical study on the oxygen vacancy (VO) in ZnO nanowires (ZnONWs), Sheetz et al. have revealed that the presence of vacancies contribute strongly to optical absorption in the visible [31]. Subsequently, a theoretical study by Zhang et al. has shown that ZnO0.875 crystal is a uniaxial crystal and exhibits some features in the low energy region, which are caused by the VO [32]. Furthermore, Tuomisto et al. have used positron annihilation spectroscopy to study the vacancy defects in ZnO crystals grown by both the conventional and contactless chemical vapor transport [29]. They have concluded that Zn vacancies (VZn) or Zn vacancy related defects are present in as-grown ZnO, irrespective of the growth method. In this paper, (Zr–Al)-codoped ZnO and possible defects (VO and VZn) introduced by (Zr, Al) dopants at substitutional-Zn sites have been considered. Theoretical studies indicated that the calculated energy gaps of
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semiconductors by HSE hybrid density functional are better agreement with experimental than by standard local and semilocal exchange correlation functionals [33–37]. In the paper, by means of HSE06 hybrid density functional calculations, the electronic structure and optical properties of (Zr–Al)-codoped ZnO have been investigated and the results may be helpful to design new ZnO optoelectronic materials. 2. Computational methods All calculations have been performed using the density functional theory (DFT) framework, implemented in the Vienna ab initio simulation package (VASP) code in MedeA software [38,39], and based on the projector augmented wave (PAW) method [40]. The valence-electron configurations for the O, Zn, Al, Zr atoms are chosen as 2s22p4, 3d104s2, 3s23p1 and 4d25s2, respectively. The structure optimization is performed using the generalized gradient approximation (GGA) of DFT employing Perdew–Burke–Ernzerhof (PBE) functional form [41] and the electronic structures and optical properties are calculated on the optimized configurations using the hybrid density functional HSE06 [42–44]. The electron wave function is expanded in plane waves to a cutoff energy of 400 eV. A 3 3 3 gamma-centered k-point grid and the tetrahedron method with Blöchl corrections for the Brillouin zone integrations are used here. In the geometry optimization process, the maximum tolerance of the force is set as 0.02 eV/Å and the electronic iterations convergence is 1 105 eV. The higher cutoff energies and denser k-points have also been calculated and discover the results changed barely, which is in accordance with other theoretical calculations [45,46]. It is known that the optical properties are determined by the dielectric function e(x) = e1(x) + ie2(x), which is mainly contributed from the electronic structures. The imaginary part e2(x) of dielectric function could be obtained from the momentum matrix elements between the occupied and unoccupied wave functions, and the real part e1(x) can be evaluated from e2(x) using the Kramer–Kronig relations [47]. Then the other optical properties, such as absorption coefficient a(x), reflectivity R(x), refractivity index n(x), and the optical conductivity r(x) can be calculated by e1(x) and e2(x) [48–50]. 3. Results and discussion 3.1. Structure optimization and relative stability The optimized lattice constants are a = b = 3.262 Å and c = 5.212 Å, which are in good agreement with the experimental values of a = b = 3.250 Å and c = 5.207 Å [51]. The result shows that the calculation parameters are reasonable. Based on the optimized wurtzite ZnO unit cell, a 2 2 2 supercell containing 32 atoms is constructed for further calculations. (Zr–Al)-codoped ZnO based on this supercell is adoped to study the effect of dopants and defects on the electronic structure and optical properties. Zhao et al [52] pointed out that Zr prefers to substitute Zn site of ZnO under most growth condition by theoretical study. Thus, we used Al and Zr atoms to substitute Zn atoms in ZnO. The calculated structure of substitutional doping system is illustrated in Fig. 1. From Fig. 1, one can see that Al and Zr are represented the substitutional doped Al and Zr atoms and neutral oxygen and zinc vacancies are represented by VO and VZn, respectively. In this paper, (Zr–Al–VO) and (Zr–Al–VZn) are used to represent the (Zr–Al)-codoped ZnO with oxygen and zinc vacancies, respectively. To understand relative stability of doped and undoped systems, it is necessary to calculate the formation energy that is estimated by the following formula [46,26].
Fig. 1. Wurtzite ZnO with 2 2 2 supercell. The yellow and red balls stand for Zn atoms and O atoms, respectively. Al and Zr are represented the substitutional doped Al and Zr atoms. The single zinc vacancy is represented by VZn and VO stands for the single oxygen vacancy (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
Ef ½Defect ¼ Etot ½Defect Etot ½ZnO þ DnZn lZn þ DnO lO DnZr lZr DnAl lAl
ð1Þ
where Etot [Defect] and Etot [ZnO] are the total energies of the supercell with and without the impurity, respectively. Dn is the number of ions (Zn, O, Zr, or Al) changed between a perfect cell and its corresponding reservoir to form a defect, and lZn, lO, lZr, and lAl are the chemical potential of Zn, O, Zr and Al, respectively. The total energies are obtained after the geometry optimization is finished. It is known that thermodynamic stability is directly related to the value of formation energy. The lower the formation energy is, the more stable the material is. From formula (1), it can be found that the formation energy is dependent on chemical potentials. However, chemical potentials are influenced by the environment. Therefore, in order to study the environment effect on chemical potentials, we consider two chemical extremes. Two upper limits lZn (Zn-rich condition) and lO (O-rich condition) are determined as the total energies of metallic Zn and gaseous O2 per atom, respectively. For ZnO, lZn and lO satisfy the relation lZn + lO = lZnO from the thermodynamic equilibrium. Under O-rich conditions, lZn is determined by the formula lZn = lZnO–lO. Under Zn-rich conditions, lO is determined by the formula lO = lZnO–lZn. Bulk aluminum and zirconium are adopted as the reference for the doping source, from which the chemical potential of Al and Zr atoms are determined. Under both Zn-rich and O-rich conditions, the formation energies of (Zr–Al)-codoped ZnO systems are obtained and the results are listed in Table 1. From the Table 1, for (Zr–Al, Zr–Al–VZn, Zr–Al–VO) codoped ZnO, the formation energies under O-rich condition are all lower than that under Zn-rich condition implying that the defective systems can be easily realized under O-rich condition. Under O-rich condition, the formation energy of (Zr–Al–VZn)-codoped ZnO is
Table 1 Impurity formation energies Ef (in eV) of (Zr–Al)-codoped ZnO systems under both Zn-rich and O-rich conditions. Configurations
(Zr–Al)-codoped ZnO (Zr–Al–VZn)-codoped ZnO (Zr–Al–VO)-codoped ZnO
Ef (eV) Zn-rich
O-rich
5.78 6.81 4.66
11.99 16.12 7.77
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16.12 eV. It is obvious that (Zr–Al–VZn)-codoped ZnO is energetically favorable structure. Under Zn-rich condition, the formation energy of (Zr–Al, Zr–Al–VZn, Zr–Al–VO) codoped ZnO are 5.78 eV, 6.81 eV and 4.66 eV, respectively. Our calculation results indicate that zinc vacancies are easier to form in (Zr–Al)-codoped ZnO than that of oxygen vacencies. Besides, the formation energies indicate that (Zr–Al, Zr–Al–VZn, Zr–Al–VO) codoped ZnO can be realized.
3.2. Electronic structures The band structure of the 32-atom supercell of pure ZnO calculated using the HSE functional is presented in Fig. S1 (Supporting Information). The calculated band gap is 2.49 eV at symmetric G point and much closer to the experimental value of 3.4 eV [53,54] than that of the PBE results (0.76 eV [2] 0.74 eV [45]). To investigate the codoping effect of (Zr–Al) on the electronic structures, band structure and the density of states (DOS) are calculated for replacing two Zn sites with Al and Zr atoms separately, as shown in Figs. 2 and 3, respectively. Compared with pure ZnO, the remarkable feature in the energy band for (Zr–Al)-codoped ZnO is that the Fermi level shifts upward into the conduction band, which indicates that the material is a ntype semiconductor. Simultaneously, the calculated band gap decreases to 1.90 eV (as shown in Fig. 2). The band structure of ((Zr–Al–VO) and (Zr–Al–VZn)) codoped ZnO are shown in Figs. S2 and S3 (Supporting Information), respectively. The calculated band gaps of ((Zr–Al–VO) and (Zr–Al–VZn)) codoped ZnO are 2.34 eV and 2.02 eV, respectively. From Fig. S2, we find that the top of valence band is at symmetric B point, and the bottom of conduction band is at symmetric G point for the band structure of (Zr–Al–VO)-codoped ZnO. The calculated band structure shows that (Zr–Al–VO)-codoped ZnO has some characteristics of indirect band gap semiconductor. Compared with (Zr–Al)-codoped ZnO, the band gap of (Zr–Al–VO)-codoped ZnO increases about 0.44 eV, indicating that the absorption threshold of (Zr–Al– VO)-codoped ZnO may has blue-shift. In addition, the band gap of (Zr–Al–VZn)-codoped ZnO increases about 0.12 eV compared with (Zr–Al)-codoped ZnO. Fig. 3 gives the partial density of states (PDOS) and total density of states (TDOS) of comparison between (Zr–Al)-codoped ZnO and pure ZnO. As shown in Fig. 3, we can see that Zr 4d states and Al 3p states overlap and mix with Zn 4s states in (Zr–Al)-codoped ZnO and the conduction band near the Fermi energy level is primarily dominated by Zr 4d, Al 3p, and Zn 4s states. Hence, the electron excitations from the occupied O 2p states to Al 3p states and Zr 4d states are reduced, leading to a
red-shift of the optical absorption edge. It is clearly seen from Fig. 3 that O 2p states are the most dominant in the energy range between 11.4 and 4.6 eV, and the Zn 3d states locate mainly in the energy range between 11.4 and 9.4 eV. It is found that the PDOS value of Zr 4d states at Fermi level is relatively large, and it makes great contribution to the TDOS of (Zr–Al)-codoped ZnO. One can see that the PDOS of Zr 4d states (Fig. 3) overlap obviously with those of Al 3p states (Fig. 3) near the Fermi level, which indicates that there exist resonance between Al 3p and Zr 4d states. After resonance between Al 3p and Zr 4d states, a portion of the Al 3p electrons could occupy the empty states of Zr 4d states, which results in electron transition occurring between Zr 4d and Al 3p. The PDOS of Zn 4s and O 2p states of (Zr–Al)-codoped ZnO shift to lower energy range compared with those of pure ZnO. Those differences caused by codoping will affect the optical properties. Fig. 4 shows the TDOS of pure ZnO, and (Zr–Al, Zr–Al–VZn, Zr–Al–VO) codoped ZnO, respectively. Compared with pure ZnO, the remarkable feature in the TDOS of (Zr–Al, Zr–Al–VZn and Zr–Al–VO) codoped ZnO is that the DOS shifts to lower energy, exhibiting the characteristic of n-tye doping. This is the result from the interaction between Al 3p or Zr 4d and Zn 4s states, which combined with conduction-band minimum. Take the position of the peak of O 2p as reference, (Zr–Al) and (Zr–Al–VZn) codoping make the energy bands move 3.34 eV and 2.39 eV toward the low energy region, respectively, while, the energy shift of (Zr–Al–VO)-codoped ZnO is 2.95 eV which is in the middle of (Zr–Al and Zr–Al–VZn) codoped ZnO.
Fig. 2. Band structure for (Zr–Al) codoped ZnO.
Fig. 4. TDOS of comparison among pure ZnO, and (Zr–Al, Zr–Al–VZn, Zr–Al–VO) codoped ZnO. The Fermi level is set to 0.
Fig. 3. DOS of comparison between (Zr–Al)-codoped ZnO and pure ZnO. The Fermi level is set to 0.
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Fig. 5 shows the PDOS of Zr 4d, Al 3p, Zn 4s and O 2p for ((Zr–Al), (Zr–Al–VO) and (Zr–Al–VZn)) codoped ZnO, respectively. The calculated PDOS of Zr 4d for ((Zr–Al), (Zr–Al–VO) and (Zr–Al–VZn)) codoped ZnO is shown in Fig. 5(a). Compared with the peak at 2.21 eV of Zr 4d states of (Zr–Al)-codoped ZnO, the peak of Zr 4d states of (Zr–Al–VZn)-codoped ZnO is separated into three obvious peaks that are located at 1.47 eV, 2.24 eV and 2.91 eV, respectively. Meanwhile, the peak of Zr 4d states of (Zr–Al–VO)codoped ZnO is separated into two obvious peaks that are located at 1.63 eV and 2.61 eV, respectively. The largest peak of Zr 4d states of (Zr–Al–VO)-codoped ZnO increases and shifts to a higher energy region. As to the PDOS of Al 3p states in Fig. 5(b), compared with (Zr–Al)-codoped ZnO, the PDOS of Al 3p states of (Zr–Al–VO)-codoped ZnO increased significantly near the Fermi-level. From Fig. 5(c and d), it is clearly seen that the peaks of PDOS of Zn 4s and O 2p of (Zr–Al–VO)-codoped ZnO increased compared with (Zr–Al)-codoped ZnO. Both the PDOS of ((Zr–Al–VO) and (Zr–Al– VZn)) codoped ZnO move slightly to higher energy simultaneously. From the above discussions, we can conclude that oxygen vacancy has more effects on the electronic structures of the (Zr–Al)-codoped ZnO, indicating that it may affect the optical properties of codoped ZnO.
3.3. Optical properties In this section, the optical properties of the pure ZnO, and ((Zr– Al), (Zr–Al–VO), (Zr–Al–VZn)) codoped ZnO are systematically discussed on the basis of the imaginary part of the dielectric function (e2(x)) and absorption coefficient (a(x)). The imaginary part of the dielectric function e2(x) for pure ZnO and ((Zr–Al), (Zr–Al–VO), (Zr–Al–VZn)) codoped ZnO are shown in Fig. 6. The imaginary part e2(x) is a critical characteristic of the optical properties for all the optical materials. In general, optical transitions between occupied and unoccupied states are caused by the electric field of the photon. Components of the optical properties corresponding to the
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Fig. 6. Imaginary part of dielectric function of pure ZnO, and (Zr–Al, Zr–Al–VO, Zr– Al–VZn) codoped ZnO.
polarization vectors perpendicular to the Z axis have been considered [1,2]. In pure ZnO, there are three main peaks at 4.39, 6.75, and 9.22 eV, respectively, and the predominant transitions in the region 11–13 eV that are consistent with experimental results [55]. The peak at 4.39 eV should mainly be caused by optical transitions between O 2p states in the highest valence band and Zn 4s states in the lowest conduction band. The peak at 6.75 eV comes from the optical transition between the Zn 3d and O 2p states, and the third peak at 9.22 eV is mainly derived from the optical transition between the Zn 3d and O 2s states. The optical properties of ((Zr–Al), (Zr–Al–VO), (Zr–Al–VZn)) codoped ZnO possess remarkable anisotropic characteristic. The imaginary part of the dielectric tensor (e2(x)) have values with six components of X, Y, Z, XY, YZ and XZ for ((Zr–Al), (Zr–Al–VO),
Fig. 5. PDOS of comparison among (Zr–Al, Zr–Al–VO, Zr–Al–VZn) codoped ZnO. The Fermi level is set to 0.
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(Zr–Al–VZn)) codoped ZnO. Because the (e2(x)) values at three components of XY, YZ and XZ are lower than other three components. Thus, these three components of XY, YZ and XZ are not considered in our discussions. In order to see the main peaks of ((Zr–Al–VO) and (Zr–Al–VZn)) codoped ZnO clearly, the vertical coordinate is set in a range from 0 eV to 60 eV (see Fig. S4, Supporting Information). Compared with pure ZnO, the imaginary parts (e2(x)) of ((Zr–Al), (Zr–Al–VO) and (Zr–Al–VZn)) codoped ZnO have sharply increased in the low energy range from 0 to 5.0 eV and the increments of ((Zr–Al) and (Zr–Al–VZn)) codoped ZnO are larger than that of (Zr–Al–VO)-codoped ZnO. Clearly, the main peaks in the low energy range for ((Zr–Al), (Zr–Al–VO), (Zr–Al–VZn)) codoped ZnO all move to a lower energy region. The peak at around 0–3 eV originates mainly from the transitions of impurity Zr 4d states or Al 3p states and Zn 4s states in the conduction band. Compared with pure ZnO, the peaks for ((Zr–Al), (Zr–Al–VO), (Zr–Al– VZn)) codoped ZnO in the middle-energy region (5.0–11.0 eV) also have increased slightly, meanwhile the peaks in the high-energy region (11.0–17.0 eV) have decreased slightly. For (Zr–Al)-codoped ZnO, three main peaks are observed which exist at 0.74, 8.87 and 12.50 eV, respectively. The first strong peak is red-shift to 0.74 eV, and this means that (Zr–Al)-codoped ZnO can realize intense visible-light absorption. For (Zr–Al–VO)-codoped ZnO, three main peaks are also observed which exist at 1.03, 9.10 and 12.20 eV, respectively. Compared with (Zr–Al)-codoped ZnO, the first strong peak is blue-shift to 1.03 eV. It is noted that the main peak of imaginary part of the dielectric function e2(x) of (Zr–Al– VO)-codoped ZnO increases significantly when the polarization vectors parallel to the Z axis (see Fig. S5, Supporting Information). The reason may be that oxygen vacancy induced (Zr–Al)-codoped ZnO lead to more changes of electronic structure. Energy levels occur deep in the gap as in the oxygen vacancy case with the gap region dominated by O 2p states. The presence of deep level O 2p states allow for electronic transitions to these levels from the valence states [31]. Especially the possibility of allowable transitions within the visible range becomes achievable. This means that oxygen vacancy can also enhance visible-light absorption. For (Zr–Al– VZn)-codoped ZnO, three main peaks exist at 0.87, 9.32 and 12.52 eV, respectively. Compared with (Zr–Al)-codoped ZnO, both the first peak and main absorption intensity of (Zr–Al–VZn)-codoped ZnO have small changes, proving that zinc vacancy affects visible-photocatalytic activity slightly. From Fig. 6, we can find that the e2(x) values of pure ZnO, and ((Zr–Al), (Zr–Al–VO), (Zr–Al–VZn)) codoped ZnO are all different in the lower energy range but tend to change slightly in the higher energy range. The reason may be that the dielectric function of ZnO are determined by interband transition from valence band O 2p level to conduction band Zn 4s, Zr 4d, and Al 3p levels in the low energy range. In the high-energy range, the dielectric functions are decided by electron transitions from Zn 3d states to O 2p states or from O 2s states to Zn 3d states [46]. Fig. 7 presents the optical absorption spectra of the ZnO systems with four different configurations. It is clearly shown that all ((Zr–Al), (Zr–Al–VO), (Zr–Al–VZn)) codoped ZnO induce the red-shift of the optical absorption edge compared with pure ZnO. The absorption threshold of ((Zr–Al), (Zr–Al–VO), (Zr–Al–VZn)) codoped ZnO shifts to the low energy range, which is decisive for the effective utilization of sunlight [56,57]. The first strong peak in 0– 3 eV region for codoped ZnO is attributed to the Zr and Al dopants. For (Zr–Al)-codoped ZnO, the first absorption peak is located at 0.89 eV. The optical absorption peak has a red-shift compared with pure ZnO, which may lead to the improvement of the visible-light photocatalytic ability. This red-shift may result from the band gap narrowing instead of the gap state excitation, and is agreement with the experimental result [27]. To realize the photocatalytic application of (Zr–Al)-codoped ZnO, lower excitation energy is needed. It is significant for the photocatalytic application of ZnO
Fig. 7. Absorption coefficient spectra of pure ZnO and (Zr–Al, Zr–Al–VO, Zr–Al–VZn) codoped ZnO.
in visible light range, because more electron–hole pairs can be produced by irradiating of lower energy. For (Zr–Al–VO)-codoped ZnO, the first absorption peak is located at 1.32 eV and has a blue-shift compared with (Zr–Al)-codoped ZnO. Meanwhile, the absorption intensity of (Zr–Al–VO)codoped ZnO is lower than that of (Zr–Al)-codoped ZnO. It is worth noting that when we consider the polarization vectors parallel to the Z axis, the first absorption peak of (Zr–Al–VO)-codoped ZnO is located at 1.03 eV and the absorption intensity is very large (see Fig. S6, Supporting Information). The results indicate that the oxygen vacancy introducing by (Zr–Al) codoping can improve the visible-light photocatalytic ability. For (Zr–Al–VZn)-codoped ZnO, the first absorption peak is located at 0.88 eV and the absorption intensity is equal to that of (Zr–Al)-codoped ZnO approximately. The zinc vacancy has weak effect to enhance the optical photocatalytic ability for (Zr–Al)-codoped ZnO. 4. Conclusions In summary, the electronic structure and optical properties of (Zr–Al)-codoped ZnO have been studied by first-principles calculations based on DFT. The electronic structures show that the Fermi levels of ((Zr–Al), (Zr–Al–VO), (Zr–Al–VZn)) codoped ZnO shift upward into the conduction band, which indicates that the materials have n-type conductivity. Compared with (Zr–Al)-codoped ZnO, both the PDOS of Al 3p and Zr 4d in (Zr–Al–VO)-codoped ZnO have changes significantly near the Fermi-level. (Zr–Al)-codoped ZnO induces the red-shift of the optical absorption edge compared with pure ZnO. The calculated optical properties indicate that (Zr–Al)codoped ZnO may lead to the improvement of the visible-light photocatalytic ability. Compared with (Zr–Al)-codoped ZnO, the first absorption peaks of (Zr–Al–VO)-codoped ZnO have a blueshift. The results indicate that oxygen vacancy introducing by (Zr–Al) codoping can also improve the visible-light photocatalytic ability and zinc vacancy has weak effect to enhance the optical photocatalytic ability for (Zr–Al)-codoped ZnO. Acknowledgments We thank the financial supports from the Major State Basic Research Development Programs (2011CBA00701, 2012CB721003), the National Natural Science Foundation of China (20933001, 20873006).
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First-principles study of electronic structure and optical properties of (Zr–Al)-codoped ZnO Jin-Hua Luo, Qun Liu, Li-Na Yang, Zhu-Zhu Sun, Ze-Sheng Li ⇑ Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry, Beijing Institute of Technology, Beijing 100081, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 3 July 2013 Received in revised form 5 September 2013 Accepted 6 September 2013 Available online 15 October 2013 Keywords: First principles (Zr–Al)-codoped ZnO Optical absorption Oxygen vacancy Zinc vacancy
a b s t r a c t The Heyd–Scuseria–Ernzerhof (HSE) hybrid density functional was used to investigate the electronic structure and optical properties of (Zr–Al)-codoped ZnO. The calculated results show that the formation energy of (Zr–Al)-codoped ZnO is low, indicating that it is the energetically favorable structure and the first absorption peak of optical absorption spectra for (Zr–Al)-codoped ZnO has a red-shift compared with pure ZnO, which may lead to the improvement on the visible-light photocatalytic ability. The zinc and oxygen vacancies introduced by (Zr–Al)-codoped ZnO have also been investigated. Through analysis the main absorption peaks of the imaginary part of the dielectric function on polarization vectors perpendicular or parallel to the Z axis, we think that oxygen vacancy introduced by (Zr–Al) codoping can also improve the visible-light photocatalytic ability and zinc vacancy has weak effect to enhance the optical photocatalytic ability compared with (Zr–Al)-codoped ZnO. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction ZnO is a unique semiconductor material with a direct wide band gap (3.37 eV) and a large exciton binding energy (60 meV), which ensures efficient ultraviolet emission [1,2]. As a result, ZnO has been widely used in photoelectric applications, such as transparent conductive oxides (TCOs) [3,4], transistors [5], diluted magnetic semiconductors [6–9], and dye-sensitized solar cells (DSSCs) [10]. In order to design novel semiconductor materials, the theoretical efforts have been devoted to study the electronic and optical properties of ZnO involved materials [1,11–13]. Doping is one of the most efficient processes to improve the structures and properties of this kind of materials [14–16]. The doping of transition metal induces drastic changes in optical, electrical, and magnetic properties of ZnO by altering its electronic structure. Khan et al. [17] demonstrated that the optical absorption spectra for Zr doped ZnO nanoparticles show the absorption in the visible region. Meanwhile, many studies indicated that Al doped ZnO is one of most hopeful candidates for replacing Tin-doped indium oxide, due to its relatively high abundance, low fabrication cost, low toxicity and thermal stability [18–21]. In addition, Al doped ZnO nanorod arrays have been found to improve the energy conversion efficiency of dye-sensitized solar cells significantly [22,23]. Recently, co-doped ZnO have received extensive studies and some research groups have demonstrated that co-doping can modify the electronic struc⇑ Corresponding author. Address: 5 South Zhongguancun Street, Haidian Zone, Beijing 10081, People’s Republic of China. Tel./fax: +86 10 68918670. E-mail address: [email protected] (Z.-S. Li). 0927-0256/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.commatsci.2013.09.021
ture and optical properties of the intrinsic ZnO efficiently [24–26]. Experimentally, Yu et al. reported that (Zr–Al)-codoped ZnO has a greater improvement on the photocatalytic activity [27]. However, compared with the widely studied donor–acceptor codoping in ZnO systems [26,28], the report on the theoretical researches about (Zr–Al)-codoped ZnO system has not been found up to date. Thus, in order to understand the relationship between the photocatalytic activity and the electronic structure and optical properties, theoretical investigation of (Zr–Al)-codoped ZnO system is indispensable. Besides, point defects have an important effect on the electronic and optoelectronic properties of semiconductor materials [29]. The experimental study by Zheng et al. has indicated that the optical and photocatalytic properties of ZnO nanocrystals are dependent on the type and concentration of oxygen defects in the synthesized ZnO nanocrystals [30]. Using theoretical study on the oxygen vacancy (VO) in ZnO nanowires (ZnONWs), Sheetz et al. have revealed that the presence of vacancies contribute strongly to optical absorption in the visible [31]. Subsequently, a theoretical study by Zhang et al. has shown that ZnO0.875 crystal is a uniaxial crystal and exhibits some features in the low energy region, which are caused by the VO [32]. Furthermore, Tuomisto et al. have used positron annihilation spectroscopy to study the vacancy defects in ZnO crystals grown by both the conventional and contactless chemical vapor transport [29]. They have concluded that Zn vacancies (VZn) or Zn vacancy related defects are present in as-grown ZnO, irrespective of the growth method. In this paper, (Zr–Al)-codoped ZnO and possible defects (VO and VZn) introduced by (Zr, Al) dopants at substitutional-Zn sites have been considered. Theoretical studies indicated that the calculated energy gaps of
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semiconductors by HSE hybrid density functional are better agreement with experimental than by standard local and semilocal exchange correlation functionals [33–37]. In the paper, by means of HSE06 hybrid density functional calculations, the electronic structure and optical properties of (Zr–Al)-codoped ZnO have been investigated and the results may be helpful to design new ZnO optoelectronic materials. 2. Computational methods All calculations have been performed using the density functional theory (DFT) framework, implemented in the Vienna ab initio simulation package (VASP) code in MedeA software [38,39], and based on the projector augmented wave (PAW) method [40]. The valence-electron configurations for the O, Zn, Al, Zr atoms are chosen as 2s22p4, 3d104s2, 3s23p1 and 4d25s2, respectively. The structure optimization is performed using the generalized gradient approximation (GGA) of DFT employing Perdew–Burke–Ernzerhof (PBE) functional form [41] and the electronic structures and optical properties are calculated on the optimized configurations using the hybrid density functional HSE06 [42–44]. The electron wave function is expanded in plane waves to a cutoff energy of 400 eV. A 3 3 3 gamma-centered k-point grid and the tetrahedron method with Blöchl corrections for the Brillouin zone integrations are used here. In the geometry optimization process, the maximum tolerance of the force is set as 0.02 eV/Å and the electronic iterations convergence is 1 105 eV. The higher cutoff energies and denser k-points have also been calculated and discover the results changed barely, which is in accordance with other theoretical calculations [45,46]. It is known that the optical properties are determined by the dielectric function e(x) = e1(x) + ie2(x), which is mainly contributed from the electronic structures. The imaginary part e2(x) of dielectric function could be obtained from the momentum matrix elements between the occupied and unoccupied wave functions, and the real part e1(x) can be evaluated from e2(x) using the Kramer–Kronig relations [47]. Then the other optical properties, such as absorption coefficient a(x), reflectivity R(x), refractivity index n(x), and the optical conductivity r(x) can be calculated by e1(x) and e2(x) [48–50]. 3. Results and discussion 3.1. Structure optimization and relative stability The optimized lattice constants are a = b = 3.262 Å and c = 5.212 Å, which are in good agreement with the experimental values of a = b = 3.250 Å and c = 5.207 Å [51]. The result shows that the calculation parameters are reasonable. Based on the optimized wurtzite ZnO unit cell, a 2 2 2 supercell containing 32 atoms is constructed for further calculations. (Zr–Al)-codoped ZnO based on this supercell is adoped to study the effect of dopants and defects on the electronic structure and optical properties. Zhao et al [52] pointed out that Zr prefers to substitute Zn site of ZnO under most growth condition by theoretical study. Thus, we used Al and Zr atoms to substitute Zn atoms in ZnO. The calculated structure of substitutional doping system is illustrated in Fig. 1. From Fig. 1, one can see that Al and Zr are represented the substitutional doped Al and Zr atoms and neutral oxygen and zinc vacancies are represented by VO and VZn, respectively. In this paper, (Zr–Al–VO) and (Zr–Al–VZn) are used to represent the (Zr–Al)-codoped ZnO with oxygen and zinc vacancies, respectively. To understand relative stability of doped and undoped systems, it is necessary to calculate the formation energy that is estimated by the following formula [46,26].
Fig. 1. Wurtzite ZnO with 2 2 2 supercell. The yellow and red balls stand for Zn atoms and O atoms, respectively. Al and Zr are represented the substitutional doped Al and Zr atoms. The single zinc vacancy is represented by VZn and VO stands for the single oxygen vacancy (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
Ef ½Defect ¼ Etot ½Defect Etot ½ZnO þ DnZn lZn þ DnO lO DnZr lZr DnAl lAl
ð1Þ
where Etot [Defect] and Etot [ZnO] are the total energies of the supercell with and without the impurity, respectively. Dn is the number of ions (Zn, O, Zr, or Al) changed between a perfect cell and its corresponding reservoir to form a defect, and lZn, lO, lZr, and lAl are the chemical potential of Zn, O, Zr and Al, respectively. The total energies are obtained after the geometry optimization is finished. It is known that thermodynamic stability is directly related to the value of formation energy. The lower the formation energy is, the more stable the material is. From formula (1), it can be found that the formation energy is dependent on chemical potentials. However, chemical potentials are influenced by the environment. Therefore, in order to study the environment effect on chemical potentials, we consider two chemical extremes. Two upper limits lZn (Zn-rich condition) and lO (O-rich condition) are determined as the total energies of metallic Zn and gaseous O2 per atom, respectively. For ZnO, lZn and lO satisfy the relation lZn + lO = lZnO from the thermodynamic equilibrium. Under O-rich conditions, lZn is determined by the formula lZn = lZnO–lO. Under Zn-rich conditions, lO is determined by the formula lO = lZnO–lZn. Bulk aluminum and zirconium are adopted as the reference for the doping source, from which the chemical potential of Al and Zr atoms are determined. Under both Zn-rich and O-rich conditions, the formation energies of (Zr–Al)-codoped ZnO systems are obtained and the results are listed in Table 1. From the Table 1, for (Zr–Al, Zr–Al–VZn, Zr–Al–VO) codoped ZnO, the formation energies under O-rich condition are all lower than that under Zn-rich condition implying that the defective systems can be easily realized under O-rich condition. Under O-rich condition, the formation energy of (Zr–Al–VZn)-codoped ZnO is
Table 1 Impurity formation energies Ef (in eV) of (Zr–Al)-codoped ZnO systems under both Zn-rich and O-rich conditions. Configurations
(Zr–Al)-codoped ZnO (Zr–Al–VZn)-codoped ZnO (Zr–Al–VO)-codoped ZnO
Ef (eV) Zn-rich
O-rich
5.78 6.81 4.66
11.99 16.12 7.77
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16.12 eV. It is obvious that (Zr–Al–VZn)-codoped ZnO is energetically favorable structure. Under Zn-rich condition, the formation energy of (Zr–Al, Zr–Al–VZn, Zr–Al–VO) codoped ZnO are 5.78 eV, 6.81 eV and 4.66 eV, respectively. Our calculation results indicate that zinc vacancies are easier to form in (Zr–Al)-codoped ZnO than that of oxygen vacencies. Besides, the formation energies indicate that (Zr–Al, Zr–Al–VZn, Zr–Al–VO) codoped ZnO can be realized.
3.2. Electronic structures The band structure of the 32-atom supercell of pure ZnO calculated using the HSE functional is presented in Fig. S1 (Supporting Information). The calculated band gap is 2.49 eV at symmetric G point and much closer to the experimental value of 3.4 eV [53,54] than that of the PBE results (0.76 eV [2] 0.74 eV [45]). To investigate the codoping effect of (Zr–Al) on the electronic structures, band structure and the density of states (DOS) are calculated for replacing two Zn sites with Al and Zr atoms separately, as shown in Figs. 2 and 3, respectively. Compared with pure ZnO, the remarkable feature in the energy band for (Zr–Al)-codoped ZnO is that the Fermi level shifts upward into the conduction band, which indicates that the material is a ntype semiconductor. Simultaneously, the calculated band gap decreases to 1.90 eV (as shown in Fig. 2). The band structure of ((Zr–Al–VO) and (Zr–Al–VZn)) codoped ZnO are shown in Figs. S2 and S3 (Supporting Information), respectively. The calculated band gaps of ((Zr–Al–VO) and (Zr–Al–VZn)) codoped ZnO are 2.34 eV and 2.02 eV, respectively. From Fig. S2, we find that the top of valence band is at symmetric B point, and the bottom of conduction band is at symmetric G point for the band structure of (Zr–Al–VO)-codoped ZnO. The calculated band structure shows that (Zr–Al–VO)-codoped ZnO has some characteristics of indirect band gap semiconductor. Compared with (Zr–Al)-codoped ZnO, the band gap of (Zr–Al–VO)-codoped ZnO increases about 0.44 eV, indicating that the absorption threshold of (Zr–Al– VO)-codoped ZnO may has blue-shift. In addition, the band gap of (Zr–Al–VZn)-codoped ZnO increases about 0.12 eV compared with (Zr–Al)-codoped ZnO. Fig. 3 gives the partial density of states (PDOS) and total density of states (TDOS) of comparison between (Zr–Al)-codoped ZnO and pure ZnO. As shown in Fig. 3, we can see that Zr 4d states and Al 3p states overlap and mix with Zn 4s states in (Zr–Al)-codoped ZnO and the conduction band near the Fermi energy level is primarily dominated by Zr 4d, Al 3p, and Zn 4s states. Hence, the electron excitations from the occupied O 2p states to Al 3p states and Zr 4d states are reduced, leading to a
red-shift of the optical absorption edge. It is clearly seen from Fig. 3 that O 2p states are the most dominant in the energy range between 11.4 and 4.6 eV, and the Zn 3d states locate mainly in the energy range between 11.4 and 9.4 eV. It is found that the PDOS value of Zr 4d states at Fermi level is relatively large, and it makes great contribution to the TDOS of (Zr–Al)-codoped ZnO. One can see that the PDOS of Zr 4d states (Fig. 3) overlap obviously with those of Al 3p states (Fig. 3) near the Fermi level, which indicates that there exist resonance between Al 3p and Zr 4d states. After resonance between Al 3p and Zr 4d states, a portion of the Al 3p electrons could occupy the empty states of Zr 4d states, which results in electron transition occurring between Zr 4d and Al 3p. The PDOS of Zn 4s and O 2p states of (Zr–Al)-codoped ZnO shift to lower energy range compared with those of pure ZnO. Those differences caused by codoping will affect the optical properties. Fig. 4 shows the TDOS of pure ZnO, and (Zr–Al, Zr–Al–VZn, Zr–Al–VO) codoped ZnO, respectively. Compared with pure ZnO, the remarkable feature in the TDOS of (Zr–Al, Zr–Al–VZn and Zr–Al–VO) codoped ZnO is that the DOS shifts to lower energy, exhibiting the characteristic of n-tye doping. This is the result from the interaction between Al 3p or Zr 4d and Zn 4s states, which combined with conduction-band minimum. Take the position of the peak of O 2p as reference, (Zr–Al) and (Zr–Al–VZn) codoping make the energy bands move 3.34 eV and 2.39 eV toward the low energy region, respectively, while, the energy shift of (Zr–Al–VO)-codoped ZnO is 2.95 eV which is in the middle of (Zr–Al and Zr–Al–VZn) codoped ZnO.
Fig. 2. Band structure for (Zr–Al) codoped ZnO.
Fig. 4. TDOS of comparison among pure ZnO, and (Zr–Al, Zr–Al–VZn, Zr–Al–VO) codoped ZnO. The Fermi level is set to 0.
Fig. 3. DOS of comparison between (Zr–Al)-codoped ZnO and pure ZnO. The Fermi level is set to 0.
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Fig. 5 shows the PDOS of Zr 4d, Al 3p, Zn 4s and O 2p for ((Zr–Al), (Zr–Al–VO) and (Zr–Al–VZn)) codoped ZnO, respectively. The calculated PDOS of Zr 4d for ((Zr–Al), (Zr–Al–VO) and (Zr–Al–VZn)) codoped ZnO is shown in Fig. 5(a). Compared with the peak at 2.21 eV of Zr 4d states of (Zr–Al)-codoped ZnO, the peak of Zr 4d states of (Zr–Al–VZn)-codoped ZnO is separated into three obvious peaks that are located at 1.47 eV, 2.24 eV and 2.91 eV, respectively. Meanwhile, the peak of Zr 4d states of (Zr–Al–VO)codoped ZnO is separated into two obvious peaks that are located at 1.63 eV and 2.61 eV, respectively. The largest peak of Zr 4d states of (Zr–Al–VO)-codoped ZnO increases and shifts to a higher energy region. As to the PDOS of Al 3p states in Fig. 5(b), compared with (Zr–Al)-codoped ZnO, the PDOS of Al 3p states of (Zr–Al–VO)-codoped ZnO increased significantly near the Fermi-level. From Fig. 5(c and d), it is clearly seen that the peaks of PDOS of Zn 4s and O 2p of (Zr–Al–VO)-codoped ZnO increased compared with (Zr–Al)-codoped ZnO. Both the PDOS of ((Zr–Al–VO) and (Zr–Al– VZn)) codoped ZnO move slightly to higher energy simultaneously. From the above discussions, we can conclude that oxygen vacancy has more effects on the electronic structures of the (Zr–Al)-codoped ZnO, indicating that it may affect the optical properties of codoped ZnO.
3.3. Optical properties In this section, the optical properties of the pure ZnO, and ((Zr– Al), (Zr–Al–VO), (Zr–Al–VZn)) codoped ZnO are systematically discussed on the basis of the imaginary part of the dielectric function (e2(x)) and absorption coefficient (a(x)). The imaginary part of the dielectric function e2(x) for pure ZnO and ((Zr–Al), (Zr–Al–VO), (Zr–Al–VZn)) codoped ZnO are shown in Fig. 6. The imaginary part e2(x) is a critical characteristic of the optical properties for all the optical materials. In general, optical transitions between occupied and unoccupied states are caused by the electric field of the photon. Components of the optical properties corresponding to the
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Fig. 6. Imaginary part of dielectric function of pure ZnO, and (Zr–Al, Zr–Al–VO, Zr– Al–VZn) codoped ZnO.
polarization vectors perpendicular to the Z axis have been considered [1,2]. In pure ZnO, there are three main peaks at 4.39, 6.75, and 9.22 eV, respectively, and the predominant transitions in the region 11–13 eV that are consistent with experimental results [55]. The peak at 4.39 eV should mainly be caused by optical transitions between O 2p states in the highest valence band and Zn 4s states in the lowest conduction band. The peak at 6.75 eV comes from the optical transition between the Zn 3d and O 2p states, and the third peak at 9.22 eV is mainly derived from the optical transition between the Zn 3d and O 2s states. The optical properties of ((Zr–Al), (Zr–Al–VO), (Zr–Al–VZn)) codoped ZnO possess remarkable anisotropic characteristic. The imaginary part of the dielectric tensor (e2(x)) have values with six components of X, Y, Z, XY, YZ and XZ for ((Zr–Al), (Zr–Al–VO),
Fig. 5. PDOS of comparison among (Zr–Al, Zr–Al–VO, Zr–Al–VZn) codoped ZnO. The Fermi level is set to 0.
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(Zr–Al–VZn)) codoped ZnO. Because the (e2(x)) values at three components of XY, YZ and XZ are lower than other three components. Thus, these three components of XY, YZ and XZ are not considered in our discussions. In order to see the main peaks of ((Zr–Al–VO) and (Zr–Al–VZn)) codoped ZnO clearly, the vertical coordinate is set in a range from 0 eV to 60 eV (see Fig. S4, Supporting Information). Compared with pure ZnO, the imaginary parts (e2(x)) of ((Zr–Al), (Zr–Al–VO) and (Zr–Al–VZn)) codoped ZnO have sharply increased in the low energy range from 0 to 5.0 eV and the increments of ((Zr–Al) and (Zr–Al–VZn)) codoped ZnO are larger than that of (Zr–Al–VO)-codoped ZnO. Clearly, the main peaks in the low energy range for ((Zr–Al), (Zr–Al–VO), (Zr–Al–VZn)) codoped ZnO all move to a lower energy region. The peak at around 0–3 eV originates mainly from the transitions of impurity Zr 4d states or Al 3p states and Zn 4s states in the conduction band. Compared with pure ZnO, the peaks for ((Zr–Al), (Zr–Al–VO), (Zr–Al– VZn)) codoped ZnO in the middle-energy region (5.0–11.0 eV) also have increased slightly, meanwhile the peaks in the high-energy region (11.0–17.0 eV) have decreased slightly. For (Zr–Al)-codoped ZnO, three main peaks are observed which exist at 0.74, 8.87 and 12.50 eV, respectively. The first strong peak is red-shift to 0.74 eV, and this means that (Zr–Al)-codoped ZnO can realize intense visible-light absorption. For (Zr–Al–VO)-codoped ZnO, three main peaks are also observed which exist at 1.03, 9.10 and 12.20 eV, respectively. Compared with (Zr–Al)-codoped ZnO, the first strong peak is blue-shift to 1.03 eV. It is noted that the main peak of imaginary part of the dielectric function e2(x) of (Zr–Al– VO)-codoped ZnO increases significantly when the polarization vectors parallel to the Z axis (see Fig. S5, Supporting Information). The reason may be that oxygen vacancy induced (Zr–Al)-codoped ZnO lead to more changes of electronic structure. Energy levels occur deep in the gap as in the oxygen vacancy case with the gap region dominated by O 2p states. The presence of deep level O 2p states allow for electronic transitions to these levels from the valence states [31]. Especially the possibility of allowable transitions within the visible range becomes achievable. This means that oxygen vacancy can also enhance visible-light absorption. For (Zr–Al– VZn)-codoped ZnO, three main peaks exist at 0.87, 9.32 and 12.52 eV, respectively. Compared with (Zr–Al)-codoped ZnO, both the first peak and main absorption intensity of (Zr–Al–VZn)-codoped ZnO have small changes, proving that zinc vacancy affects visible-photocatalytic activity slightly. From Fig. 6, we can find that the e2(x) values of pure ZnO, and ((Zr–Al), (Zr–Al–VO), (Zr–Al–VZn)) codoped ZnO are all different in the lower energy range but tend to change slightly in the higher energy range. The reason may be that the dielectric function of ZnO are determined by interband transition from valence band O 2p level to conduction band Zn 4s, Zr 4d, and Al 3p levels in the low energy range. In the high-energy range, the dielectric functions are decided by electron transitions from Zn 3d states to O 2p states or from O 2s states to Zn 3d states [46]. Fig. 7 presents the optical absorption spectra of the ZnO systems with four different configurations. It is clearly shown that all ((Zr–Al), (Zr–Al–VO), (Zr–Al–VZn)) codoped ZnO induce the red-shift of the optical absorption edge compared with pure ZnO. The absorption threshold of ((Zr–Al), (Zr–Al–VO), (Zr–Al–VZn)) codoped ZnO shifts to the low energy range, which is decisive for the effective utilization of sunlight [56,57]. The first strong peak in 0– 3 eV region for codoped ZnO is attributed to the Zr and Al dopants. For (Zr–Al)-codoped ZnO, the first absorption peak is located at 0.89 eV. The optical absorption peak has a red-shift compared with pure ZnO, which may lead to the improvement of the visible-light photocatalytic ability. This red-shift may result from the band gap narrowing instead of the gap state excitation, and is agreement with the experimental result [27]. To realize the photocatalytic application of (Zr–Al)-codoped ZnO, lower excitation energy is needed. It is significant for the photocatalytic application of ZnO
Fig. 7. Absorption coefficient spectra of pure ZnO and (Zr–Al, Zr–Al–VO, Zr–Al–VZn) codoped ZnO.
in visible light range, because more electron–hole pairs can be produced by irradiating of lower energy. For (Zr–Al–VO)-codoped ZnO, the first absorption peak is located at 1.32 eV and has a blue-shift compared with (Zr–Al)-codoped ZnO. Meanwhile, the absorption intensity of (Zr–Al–VO)codoped ZnO is lower than that of (Zr–Al)-codoped ZnO. It is worth noting that when we consider the polarization vectors parallel to the Z axis, the first absorption peak of (Zr–Al–VO)-codoped ZnO is located at 1.03 eV and the absorption intensity is very large (see Fig. S6, Supporting Information). The results indicate that the oxygen vacancy introducing by (Zr–Al) codoping can improve the visible-light photocatalytic ability. For (Zr–Al–VZn)-codoped ZnO, the first absorption peak is located at 0.88 eV and the absorption intensity is equal to that of (Zr–Al)-codoped ZnO approximately. The zinc vacancy has weak effect to enhance the optical photocatalytic ability for (Zr–Al)-codoped ZnO. 4. Conclusions In summary, the electronic structure and optical properties of (Zr–Al)-codoped ZnO have been studied by first-principles calculations based on DFT. The electronic structures show that the Fermi levels of ((Zr–Al), (Zr–Al–VO), (Zr–Al–VZn)) codoped ZnO shift upward into the conduction band, which indicates that the materials have n-type conductivity. Compared with (Zr–Al)-codoped ZnO, both the PDOS of Al 3p and Zr 4d in (Zr–Al–VO)-codoped ZnO have changes significantly near the Fermi-level. (Zr–Al)-codoped ZnO induces the red-shift of the optical absorption edge compared with pure ZnO. The calculated optical properties indicate that (Zr–Al)codoped ZnO may lead to the improvement of the visible-light photocatalytic ability. Compared with (Zr–Al)-codoped ZnO, the first absorption peaks of (Zr–Al–VO)-codoped ZnO have a blueshift. The results indicate that oxygen vacancy introducing by (Zr–Al) codoping can also improve the visible-light photocatalytic ability and zinc vacancy has weak effect to enhance the optical photocatalytic ability for (Zr–Al)-codoped ZnO. Acknowledgments We thank the financial supports from the Major State Basic Research Development Programs (2011CBA00701, 2012CB721003), the National Natural Science Foundation of China (20933001, 20873006).
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