ARTICLE pubs.acs.org/JPCC
Structural, Electronic, and Optical Properties of Ag-Doped ZnO Nanowires: First Principles Study Yanlu Li,† Xian Zhao,† and Weiliu Fan†,‡,* †
State Key Laboratory of Crystal Materials, and ‡Department of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China
bS Supporting Information ABSTRACT: First-principles calculations have been performed to determine the effects of Ag doping to the structural, electronic, and optical properties of ZnO NWs. The calculated formation energies are very low for Ag dopants at substitutional-Zn sites (both under low and high Ag concentration), but rather high at substitutional-O and interstitial sites under O-rich conditions. The AgZn and 2AgZn defects all prefer the edge of the NW and the formation energy of 2AgZn in the favorable O-rich conditions is only 0.40 eV. The calculated acceptor levels of AgZn and 2AgZn ones are 0.60 and 0.44 eV respectively, indicating Ag may be a good candidate for producing p-type ZnO NW. From the optical properties calculations, strong absorptions have been found in the visible-light region for both the 2AgZn and AgO-doped ZnO NWs. It provides evidence that, except for the usage as short-wavelength optoelectronic devices, Ag-doped ZnO NWs could also display potential application for photocatalysis due to the increase of the visible-photocatalytic activity.
’ INTRODUCTION Zinc oxide (ZnO), a wide direct band-gap (3.37 eV) semiconductor with a large excitation binding energy of 60 meV, is known to be one of the most important functional oxides with near-UV emission, transparent conductivity, piezoelectricity, and so forth.1-4 Beyond the bulk materials and thin films that have been widely studied and developed, considerable research interests have been attracted on the 1D ZnO nanowires (NWs) due to their unique properties and novel applications in the photovoltaics, gas sensors, photocatalysis, and piezoelectricity.5-7 Their reduced dimensionality and size can facilitate more efficient carrier transport due to decreased grain boundaries, surface defects and disorders, and discontinuous interfaces. Generally, the optoelectronic behaviors of semiconductors are dominated by the defects and impurities. It is currently an important issue for impurity incorporation in ZnO to develop the electronic applications. In particular, the p-type doping of ZnO is still a difficult problem and of great importance.8-10 Many efforts have been made to obtain p-type ZnO by using different dopant sources. Early research showed that Ag incorporation in ZnO reduces donor density, which means that Ag may be an effective acceptor in ZnO.11 Some recent experimental studies confirmed the p conductivity in Ag-doped ZnO.12-14 Simultaneously, a theoretical study by Yanfa Yan15 showed that Ag is expected to be an effective acceptor for producing p-type ZnO. The ionization energy of Ag is reported to be 0.4 eV,16 which confirms potential application of Ag as an acceptor in ZnO. It also points out that Ag incorporation at the interstitial and oxygen sites are not efficient due to the high formation energies. In fact, Ag is not only a good electric conductor with relatively low optical absorption coefficient r 2011 American Chemical Society
in the visible region but also an important optical material in the visible region and the near-infrared region. Ag gains great effects on the photoactivity of semiconductor photocatalysis - Ag incorporation in ZnO could greatly improve the photocatalytic degradation activity of ZnO.17-19 A recent report20 showed that the photocatalytic activity of Ag-doped ZnO photocatalyst was five times higher than the unmodified one using sunlight. Although there are some works on the synthesis, optoelectrical properties, and photocatalysis of Ag-doped ZnO bulk and films, detailed studies on the effect of Ag modification in ZnO NWs are still very scarce. The 1D ZnO NWs will have distinct size- and shape-dependent optoelectric and catalytic properties when compared with the bulk ZnO. And the active sites can distribute on the surface uniformly due to the high surfacevolume ratio of ZnO NWs. Thus, theoretical understanding of the doping effect and mechanism at the nanoscale, which are related to the surface effect and quantum confinement effect, is of significant importance for better utilizing ZnO NWs in practical applications such as electric devices and photocatalysts. In this article, we systematically investigate the structural, electronic, and optical properties of Ag-doped ZnO NWs using firstprinciples calculations based on DFT. The stability of different doping states and the proper growth atmosphere conditions, which is of benefit for p-type or photocatalytic applications have been illustrated. It can provide some valuable information on the
Received: October 14, 2010 Revised: November 27, 2010 Published: February 14, 2011 3552
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synthesis of Ag-doped ZnO NW materials with specific applications.
’ COMPUTATIONAL METHODS Spin-polarized first-principles DFT calculations were performed using the DMol3 package,21,22 with the generalized gradient approximation (GGA) of Becke-Lee-Yang-Parr (BLYP)23,24 exchange-correlation functional. All electron treatment for silver, zinc, and oxygen atoms and a double numeric basis set including d polarization function (DNP) were adopted in all the computations. The orbital cutoff was set to 4.5 Å. A 1 1 4 Monkhorst-Pack mesh25 was used for k-point sampling for atomic force relaxation, whereas a dense 1 1 8 k-point mesh was used for the computation of the total energy, electronic, and optical properties. The supercell contained a vacuum region of ∼12 Å perpendicular to the NW axis, which is sufficiently large to reduce the interactions between adjacent NWs under periodic boundary conditions. All atomic coordinates, as well as the lattice constant along the NW axis, were fully relaxed with the force on each atom in all cases reduced to less than 0.03 eV/ Å. The electronic and optical properties were calculated on the basis of the optimized supercell. The calculated bulk lattice parameters of ZnO are a = 3.278 Å, c = 5.295 Å, within 2% larger than the experimental values of a = 3.250 Å, c = 5.207 Å.26 The GGA-BLYP calculation shows a direct band gap of 1.22 eV at the Γ-point for bulk ZnO, which is close to previous GGA results27,28 but lower than the experimental value of 3.30 eV.29 It is well-known that DFT typically underestimates the energy gap30 and the DFTþU method is considered to provide more accurate predictions of the band gap when containing transition metals. However, recent research by Volnianska et al.16 showed that the þU corrections for Zn and Ag only affect eigenenergies of the Ag-doped ZnO system by about 0.05 and 0.01 eV, which could be neglected due to the shallow character of the acceptors. Besides, because only the relative positions of the occupied states and empty states need taking into account, the DFT method is still widely accepted to depict the defect states in the electronic structure calculations, and this gives reasonable explanation for the experimental results.16,27,28 Thus, the basic results and explorations reported here should not be changed by using the GGA-BLYP approach. We could describe the interaction of a photon with the electrons in terms of time-dependent perturbations of the groundstate electronic states. The electric field of the photon leads to the transition between occupied and unoccupied states, including plasmons and single-particle excitations. The excitation spectra can be described as a joint density of states between the valence and conduction bands. The optical properties are determined by the dielectric function ε(ω) = ε1(ω) þ iε2(ω), which is mainly contributed from the electronic structures. The imaginary part ε2(ω) of dielectric function could be obtained from the momentum matrix elements between the occupied and unoccupied wave functions, and the real part ε1(ω) can be evaluated from ε2(ω) using the Kramer-Kronig relations:31 ε2 ðωÞ ¼
∑∈ ν ∑k jÆΨckj μ∧ rjΨvk æj2 δ½Ekc - Ekv - pω
2π2 e2 Ωε0 i ∈ c:f
ð1Þ Z ¥ 2 ω02 ε2 ðω0 Þ ε1 ðωÞ ¼ 1 þ dω0 02 π 0 ω - ω2
ð2Þ
Figure 1. Top (a) and side (b) views of an H-passivated ZnO NW. Zn, O, and H atoms are denoted with gray, red, and white balls, respectively. Different substitutional Zn and O sites are denoted in (a). Different octahedral and tetrahedral interstitial sites are marked as “*” and “þ” in (b), respectively.
Arising from ε1(ω) and ε2(ω), all of the other optical properties, such as reflectivity R(ω), absorption coefficient R(ω), refractive index n(ω), and energy-loss spectrum L(ω) can be calculated:32,33 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ε ðωÞ þ jε ðωÞ - 12 1 2 RðωÞ ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð3Þ ε1 ðωÞ þ jε2 ðωÞ þ 1 pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ω½ ε21 ðωÞ þ ε22 ðωÞ - ε1 ðωÞ1=2
ð4Þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi nðωÞ ¼ ½ ε21 ðωÞ þ ε22 ðωÞ þ ε1 ðωÞ1=2 = 2
ð5Þ
LðωÞ ¼ ε2 ðωÞ=½ε21 ðωÞ þ ε22 ðωÞ
ð6Þ
RðωÞ ¼
’ RESULTS AND DISCUSSION A. Model Structures and Relative Stability of Ag-Doped ZnO NW. We first investigate the structural properties of defect-
free ZnO NWs prior to studying the defects. The ZnO NWs studied have a hexagonal cross section enclosed by (1010) facets, with the wire axis along the polar [0001] direction (defined as the z direction), as shown in Figure 1. The NW is modeled by a periodical supercell, and each ZnO NW supercell contains 48 Zn and 48 O atoms with a diameter about 10 Å. We use pseudohydrogen atoms to passivate dangling bonds on the surface of NW. In the NW core, the Zn-O bond lengths are 1.974-1.990 Å, which are quite close to those in bulk ZnO. Whereas the surface structure of NW has been reconstructed: Zn atoms move outward while O atoms inward, resulting in obvious radial displacement. The Zn-O bond lengths at surface are elongated to 2.014-2.166 Å, whereas the Zn-O bond lengths between the surface atoms and the core atoms are shortened to 1.946-1.984 Å. The preferable sites for Ag dopants are determined by estimating the formation energies of different doping configurations. Amounting to 2.08% Ag concentration, three kinds of doping models are considered, including one substitutional Ag at Zn site (AgZn), one substitutional Ag at O site (AgO), and one Ag at interstitial site (Agi). Their positions are depicted in Figure 1. 3553
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Table 1. Formation Energies Ef (in eV) of the Defects at Different Positions of ZnO NW in Both Zn-rich and O-rich Conditions Ef (eV) defects AgZn
AgO
Agi
positions
Zn-rich
O-rich
Zns
3.32
0.91
Znsub
4.10
1.68
Znc
4.31
1.89
Os
4.33
4.75
Osub Oc
4.13 4.55
4.55 4.97
IOs
6.14
6.57
IOc
4.87
5.29
ITc
6.40
6.83
Figure 2. Side and top views of partial geometries for AgZn (a), AgO (b), and Agi (c) -doped ZnO NW. The light-blue spheres represent Ag atoms.
Part a of Figure 1 shows three different Zn sites that can be substituted by Ag, namely, the surface H-passivated Zns atom, the surface four-coordinated Znsub atom, and the four-coordinated Znc atom in the core. The substitution of O atom with Ag shares the same case. In the wurtzite ZnO structure, there are two highsymmetry interstitial sites, namely, the octahedral (IO) and tetrahedral (IT) ones. As Ag could not stably exist at the tetrahedral site of the surface, we consider Ag incorporation in the octahedral interstitial sites at the surface and core (IOs and IOc) as well as the tetrahedral interstitial site at the core (ITc), as shown in part b of Figure 1. The formation energy of these defects in the charge neutral state is defined by Ef ¼ EðAg : ZnOÞ - EðZnOÞ þ ΔnZn μZn þ ΔnO μO - ΔnAg μAg
ð7Þ
where E(Ag:ZnO) and E(ZnO) are the total energies of the supercell with and without the impurity, respectively. Δn is the number of ions (Zn, O, or Ag) changed between a perfect cell and its corresponding reservoir to form a defect; and μ is the chemical potential of Ag, O, or Zn. The formation energy is strongly dependent on the reservoirs used to determine the chemical potentials. Two chemical extremes are considered. The upper limits for μZn (Zn-rich conditions) and μO (O-rich conditions) are determined as the total energies of metallic Zn and gaseous O2 per atom, respectively. The chemical potentials of Zn under O-rich conditions and O under Zn-rich conditions obtained from the thermodynamic equilibrium μZn þ μO ¼ μZnONW
ð8Þ
Thus, μZn (O-rich) = μZn (metal) þ ΔHf(ZnONW) and μO (Zn-rich) = (1/2)μO2 þ ΔHf(ZnONW), where the heat of formation of the ZnO NW ΔHf(ZnONW) = μZnONW - μZn (metal) - (1/2)μO2. The reservoir chosen for Ag is Ag2O, and the chemical potential of Ag in the two limiting oxygen environments are obtained from the equilibrium conditions 2μAg þ μO ¼ μAg2 O
ð9Þ
The calculated formation energies for single Ag dopant in different positions are summarized in Table 1, and the most stable structures for AgZn, AgO, and Agi doping are given in
Figure 3. Side views of six configurations of 2AgZn-doped ZnO NW. The big light-blue spheres represent Ag atoms.
Figure 2. It indicates that Ag dopant prefers to substitute Zn site both under Zn-rich and O-rich conditions, and Ag substitution of Zns atom is the most energetically favorable structure. It is noted that the formation energy of Ag substitution of Zns atom (0.91 eV) is extreme low under O-rich conditions that Ag may hardly dope in any other positions of ZnO NW under O-rich environment. We find that Ag impurity may also substitute the Osub atom under Zn-rich conditions when Ag and Zn are all sufficient. The AgO defect makes the neighboring Zn atoms move outward seriously due to the much larger ionic radius than O (part b of Figure 2). The Agi defect also companies with serious deformation of the NW surface (part c of Figure 2), and the formation energy of the most stable Agi configuration is so high that Ag could hardly incorporate in the interstitial site of ZnO NW in any conditions. Therefore, we only consider the electronic and optical properties of the most stable AgZn- and AgO-doped ZnO NW with 2.08% Ag concentration below. Higher dopant concentration of 4.16% has been modeled by substituting two Zn atoms with two Ag atoms (2AgZn) in the 96 atom ZnO NW supercell. There are many configurations for 2AgZn in ZnO NW, but the results of AgZn dopant show that Ag substitution in the surface is easier than in the core of NW. So Figure 3 shows six different 2AgZn defect structures proposed: configurations A, B, and C correspond to the Ag replacements of nearest-neighbor Zns-Zns, Zns-Znsub, and Zns-Znc atoms in the same (1010) layer; configurations D and E correspond to the Ag replacements of nearest-neighbor Zns-Zns and Zns-Znsub 3554
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Table 2. Formation Energies Ef (in eV) of Double Ag Substitution at Different Zn Sites of ZnO NW in Both Zn-rich and O-rich Conditions Ef (eV) configurations
Zn-rich
O-rich
A
5.23
0.40
B
6.86
2.03
C
6.95
2.12
D
5.87
1.04
E F
7.27 5.91
2.44 1.08
Figure 5. (A) DOS and (B) PDOS of (a) pure ZnO NW, (b) AgZndoped ZnO NW, (c) 2AgZn-doped ZnO NW, and (d) AgO-doped ZnO NW.
Figure 4. Spin-polarized electronic band structures of the pure and Agdoped ZnO NW. The up-spin and down-spin channels have been distinguished in red and blue colors. The Fermi energy is set to zero.
atoms in the different (1010) layers; configuration F corresponds to the Ag replacement of second-nearest-neighbor Zns-Zns atoms in the same (1010) layer. The calculated formation energies in Table 2 show that all of the 2AgZn configurations are easier to form under O-rich conditions. Configuration A is the most stable structure both in Zn-rich and O-rich conditions, which demonstrates that Ag atoms tend to form clusters on the NW surface. Similar results have been reported by Li et al.34 in Cr-doped ZnS NW system. It is noted that the lowest formation energy of 2AgZn (0.4 eV) is lower than that of AgZn under O-rich conditions, meaning that Ag substitution in ZnO NW with higher concentration can be obtained much easier. B. Electronic Structures of Ag-Doped ZnO NW. To investigate the influence of Ag dopants on the modification of the electronic structures, we have calculated the spin-polarized band structures and density of states (DOS) of AgZn-, 2AgZn-, and AgO-doped configurations with pure ZnO NW for comparison, as shown in Figures 4 and 5. It can be seen that the calculated band gap of the pure ZnO NW is 2.72 eV, close to other calculated value of 2.24 eV35 showing some improvement of the band-gap underestimation. The band gap of ZnO NW is larger than that of the bulk material (1.22 eV here) due to the quantum confinement effect. When introducing an AgZn defect, a single gap state is formed about 0.6 eV above the valence band maximum (VBM), as shown in part b of Figure 4, indicating the achievement of the p-type ZnO NW. However, this acceptor level locates about 0.2 eV higher than that in the bulk ZnO.15,16 This deviation may be mainly from the quantum confinement effect of the NW structure, and thus, Ag acceptor shows less effective in AgZn-doped ZnO NW than in the bulk. In the 2AgZndoped ZnO NW system, it follows from part c of Figure 4 that the host valence band (VB) has been surpassed and three gap states
been introduced above the VB. The lowest defect level has been occupied and locates about 0.24 eV above the host VBM. The other two gap states are unoccupied and located at 0.44 and 1.03 eV above the Fermi level. The lowest acceptor level in 2AgZndoped ZnO NW is shallower than that in AgZn-doped system. The smallest energy difference among the host VB and defect states in 2AgZn-doped structure are only 0.24, 0.20, and 0.59 eV, respectively. All of these results illustrate that 2AgZn-doped ZnO NW may demonstrate better p-type behavior than the AgZndoped system. Therefore, it may be available to obtain the p-type ZnO NW by incorporating Ag under O-rich conditions. The partial density of states (PDOS) in part a of Figure 5 shows that the top of the valence band of ZnO NW is mainly built up from O 2p and Zn 3d states and the conduction band (CB) is mainly from Zn 4s states. For the AgZn-doped ZnO NW, from the PDOS in part b of Figure 5, Ag-induced acceptor state contains a large contribution of Ag 4d states together with small components of O 2p and Ag 5s states, demonstrating the occurrence of the O 2p-Ag 4d hybridization. This d-character induced by the Ag dopant can be clearly seen from the electron density difference map of the (1010) plane containing Ag atom in Figure S1 of the Supporting Information. The electrons may excite from VB to the unoccupied gap state and lead to a new optical absorption in the visible-light region. As discussed above, with Ag dopant concentration increasing (2AgZn-doped model), more impurity states have been introduced. The occupied defect state is mainly from Ag 5s states with a small mixing of O 2p states, and the other two gap states are mainly composed by Ag 4d states with small contributions of Ag 5s and O 2p states (part c of Figure 5 and part b of 6). The electron excitation from VB to the gap states above the Fermi level might lead to a significant red shift of the absorption edge with a maximum about 2.28 eV, which means visible absorption to some extent. From this perspective, the introduction of Ag dopants into ZnO NW is benefit for the photocatalytic application in practice by extending the optical absorption edge to the visible-light region. As for the substitutional Ag to O-doped model, the band structure in part d of Figure 4 shows a relatively larger decline of 3555
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Figure 7. Imaginary part (a) and real part (b) of dielectric function of Ag-doped ZnO NW as compared with those of pure ZnO bulk and NW.
Figure 6. Absorption coefficient spectra of Ag-doped ZnO NW as compared with those of pure ZnO bulk and NW.
the CB with a spread of the VB to lower energy. At the same time, the AgO defect fulfills the Ag 5s gap states, shifting the Fermi level upward above these localized states and into the CB. The result is that AgO-doped ZnO NW shows typical characters of n-type semiconductor. The introduced donor levels lie at about 0.44 and 0.97 eV below the conduction band minimum (CBM), respectively. Such n-type behavior will decrease the p-type doping efficiency induced by AgZn defect, so the synthesis of Ag-doped ZnO NW for p-type application should under O-rich conditions to reduce the formation of AgO defect. However, from the PDOS in part d of Figure 5 we can find a large part of hybridization between Ag 5s and Zn 4p states at about 1.0 eV above the host VBM of ZnO NW. This can be confirmed by the obvious s-character of Ag atom and p-character of surrounding Zn atoms in the electron density difference map of AgO-doped ZnO NW in part c of Figure S1 of the Supporting Information. The electron excitations from Ag 5s or Ag 5p occupied states to the states above the Fermi level are reduced by a minimum of 0.6 eV with respect to perfect ZnO NW, leading to a red shift of the optical absorption edge. Moreover, the Ag 5s gap states above the host VBM supply a bridge for electron excitation and thus enhance the photocatalysis efficiency. Therefore, it can be follows that AgO defect is benefit for improving visible photocatalytic properties of ZnO NW. C. Optical Properties of Ag-Doped ZnO NW. In this section, we systematically discussed the optical properties of Ag-doped ZnO NW on the basis of the absorption coefficient and dielectric function with comparisons of the optical spectra of pure ZnO bulk and NW. Components of the optical properties corresponding to the polarization vectors perpendicular to the z axis have been considered. A scissors approximation operation of 2.08 eV has been employed to fix the discrepancy between the calculated and measured band-gap value of pure ZnO. The absorption coefficient R(ω) of all considered configurations have been plotted in Figure 6. It can be seen that the absorption edge of pure ZnO NW (4.44 eV) has a blueshift of 1.08 eV to that of ZnO bulk due to the quantum confinement effect. In the AgZn model, the absorption edge has only about 0.06 eV red shift to that of pure ZnO NW and is still located at the UV region. This little red shift may result from the slight band gap narrowing instead of the gap state excitation, and is agreement with the experimental red shift of 0.04 eV for Ag-doped ZnO film.36 When the Ag substitution concentration increasing to 4.16% (2AgZn), the main absorption edge has a further red shift about 0.08 eV to
that of pure ZnO NW, which is also come from the band gap narrowing and reproduces the experimental red shift of 0.06 eV.36 It is noted that another absorption peak at about 3.0 eV has been enhanced with the increase of Ag substitution concentration. It further red shifts the absorption edge of the 2AgZn-doped ZnO NW to 2.60 eV, which means the improvement of the visible-light photocatalytic ability. Combined with the PDOS in part c of Figure 5, the electron excitation from VB to the lower unoccupied Ag 4d gap states may induce significant red shift of 1.84 eV, which corresponds to the absorption at 3.0 eV. When the Ag dopant concentration continues to increase, the excitation from VB to the higher Ag 4d gap state (shown in part c of Figure 5) will lead to a new absorption peak at about 3.7 eV. However, the absorption coefficient curve of AgO-doped ZnO NW indicates a strong absorption in the visible-light region, whose intensity can be comparable to the main absorption peak in the UV region. It means that, under lower AgO dopant concentration, the material can achieve effect visible-photocatalytic activity. The absorption edge has been red-shifted by 2.2 eV as compared with pure ZnO NW, and is mainly from the excitation from the occupied Ag 5s gap states to the CBM, as seen in part d of Figure 4 and part d of Figure 5. As a result, substitutional-Ag to Zn with high concentration and to O atom would be all effective methods to achieve visible-photocatalysis of ZnO NW. However, it is noted that the main absorption peaks of Ag-doped ZnO NWs in the UV region have been seriously reduced as compared with that of ZnO bulk material, which would limit their applications in UV optoelectronic devices. The imaginary part ε2(ω) and real part ε1(ω) of the dielectric function for Ag-doped ZnO NW are shown in Figure 7. It can be seen from part a of Figure 7 that, compared to ZnO bulk material, all of the peaks of ε2(ω) have been weaken when forming 1D NW structures. The line shape of ε2(ω) is almost the same for all of the Ag-doped ZnO NW structures in the high energy range (>6 eV), and the different Ag doping configurations mainly affect the optical properties in the low-energy region. Because of the low dopant concentration, AgZn defect shows little influence on the ε2(ω) of ZnO NW, and the electronic transition from the hybridization of O 2p and Ag 4d states in the VBM to Zn 4s states in CBM leads to the lowest peak at 4.8 eV (part b of Figure 5). With the AgZn concentration increasing, another small peak appears at 3.1 eV, and it is attributed to the electronic transition from the hybridization gap states of O 2p and Ag 4d to Zn 4s states in CBM (part c of Figure 5). Similarly, the ε2(ω) curve of AgO-doped ZnO NW shows much difference with others in the low-energy region. The additional peak at 2.7 eV stems from the electronic transition from the Ag 5s gap state to the unoccupied Ag 5p and Zn 4p states, whereas the peak at 4.2 eV is from the electronic transition from Ag 4d state in the VB to the Ag 5p and Zn 4p states in the CB. 3556
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The Journal of Physical Chemistry C From the ε1(ω) curve in part b of Figure 7, we can get that ZnO bulk material displays excellent dielectric behavior and behaves like a metallic property above 11.2 eV. Quite differently, the peaks of ε1(ω) for all ZnO NW configurations are much weaker, and, more importantly, they are all positive and dielectric in the whole energy range. It means that the ZnO bulk material may transform from metallic to dielectric property above 11.2 eV in the process of becoming NW. The red shift phenomenon is in agreement with the electronic structures presented in Figures 4 and 5. The calculated static dielectric constant is 2.80 for ZnO bulk, which agrees quite well with the experimental value of 2.702 for ZnO film at 295 K.37 It also follows from part b of Figure 7 that the static dielectric constants ε0 are 1.60, 1.60, 1.64, and 1.82 for pure, AgZn, 2AgZn, and AgO-doped ZnO NW, respectively. Although 1D ZnO NWs have significantly reduced the static dielectric constants by about 1.0 eV, AgO defect may slight improve this phenomenon as compared to AgZn defects. Some other optical properties, such as reflectivity and energy-loss function of Ag-doped ZnO NW, have also been calculated and given in the Supporting Information. Similar characters have been reflected from these spectra.
’ CONCLUSIONS In summary, first-principles calculations have been performed to study the geometric, electronic, and optical properties of different defects in Ag-doped ZnO NW. The results show that substitutional-Ag to surface-Zn atom is the most energetically favorable. With the increase of the doping concentration, a small amount of AgO substitutional defects may appear under Zn-rich conditions, and Ag dopants prefer to substitute two nearest surface-Zn atoms in the same cross section of NW under O-rich conditions. It follows that AgZn- and 2AgZn-doped ZnO NWs all can achieve p-type doping, whereas the AgO-doped structure exhibits n-type character. Therefore, one should synthesize Agdoped ZnO NW for p-type applications under O-rich conditions. From the point of photocatalysis, 2AgZn and AgO defects all can improve the visible photocatalytic properties of ZnO NW by introducing another absorption peak related to Ag gap state excitation in the visible-light region. It is noted that the AgO defect is more excellent in visible photocatalysis - stronger visible-light absorption with further red shift as compared to 2AgZn-doped ZnO NW. ’ ASSOCIATED CONTENT
bS
Supporting Information. Atomic configurations and the corresponding electron density difference contour maps of the (1010) planes, which contain Ag defects in ZnO NW; reflectivity and energy-loss function of Ag-doped ZnO NW. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: 86-531-88366330, Fax: 86-531-88365174, E-mail: fwl@sdu. edu.cn.
’ ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (Grant No. 50802056), 973 Program of China (Grant No. 2009CB930103), Youth Scientist (Doctoral) Foundation of Shandong Province of China (Grant No.
ARTICLE
BS2009CL038), and the Independent Innovation Foundation of Shandong University (Grant No. 2009TS016).
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