Mechanism of Ag Doping in ZnO Nanowires by Electrodeposition

Article pubs.acs.org/JPCC

Mechanism of Ag Doping in ZnO Nanowires by Electrodeposition: Experimental and Theoretical Insights M. A. Thomas,†,‡ W. W. Sun,†,‡ and J. B. Cui*,† †

Department of Physics and Astronomy, and ‡Department of Applied Science, University of Arkansas at Little Rock, Little Rock, Arkansas 72204, United States ABSTRACT: A range of complementary techniques was used to explore the physical mechanisms of Ag doping in ZnO nanowires obtained by a low-temperature electrochemical process. Cyclic voltammetry analysis was employed to demonstrate the ability of Ag to modify the electrochemistry and in turn the ZnO growth environment generating amenable conditions for p-type doping. Both X-ray photoelectron spectroscopy and calculations using density functional theory (DFT) showcase that the principal Ag impurity in the nanowires is Ag substitution for Zn (AgZn). The calculations also indicate that AgZn doping forms an impurity band because of Ag 4d and O 2p orbital interactions shifting the Fermi level toward the valence band. Electrical characterization of the Ag-doped nanowires confirms the observations in the DFT calculations. It was also found that the formation of Ag acceptors is favorable under O rich growth conditions which can be experimentally tuned. The combination of experimental and theoretical studies performed in this work helps us to understand the Ag-doping mechanism in low-temperature growth opening up possible directions toward highly conductive p-type ZnO for advanced optoelectronic applications.

I. INTRODUCTION Progress toward device applications of ZnO in the area of electronics and optoelectronics is reliant on the ability to control its fundamental optical and electrical properties. Most especially, device development is currently inhibited by the absence of a viable method for obtaining p-type ZnO. A dominant dopant or growth process for fabricating p-type ZnO remains to be seen leaving much room for continued exploration of innovative routes toward p-type doping success in ZnO. Low-temperature growth processes for ZnO materials are becoming increasingly studied because of their low cost and scale-up potential1−4 and may create unique conditions for doping ZnO that more common, high-temperature growth techniques struggle to provide.5−8 The electrochemical growth of ZnO in aqueous solution falls into this category and has previously shown success in doping ZnO for a variety of applications.9−12 Among various potential dopant materials, Ag has demonstrated its suitability for p-type doping of ZnO. So far, Agdoped ZnO nanostructures and thin films have been explored by techniques including chemical vapor deposition,13,14 pulsed laser deposition,15−17 sputtering,18−20 and chemical solution methods.21,22 In many cases of the high-temperature methods, the Ag-doped materials showcased p-type conductivity.13−16,18,20 We have recently made strides in doping ZnO nanowires with Ag utilizing low-temperature electrochemical methods.23−25 In our previous work, the incorporation of Ag into the ZnO nanowires and their p-type properties were demonstrated by both optical and electrical measurements. © 2012 American Chemical Society

Strong acceptor-related emissions were found in low-temperature photoluminescence (PL) measurements,24,25 and the changes in the optical behavior of the Ag-doped nanowires correlated well with their observed p-type electrical properties.25 It is evident that electrochemical doping is a promising route for achieving p-type ZnO doped with Ag. However, much effort is still needed to realize highly conductive p-type ZnO nanowires for practical device applications. A sound understanding of the doping mechanism is critical for accomplishing this goal. Unfortunately, the underlying physics behind p-type doping of ZnO with Ag and other dopants is still unclear. Particularly, the chemical state of Ag in ZnO, its impact on ZnO’s electronic structure, and the chemistry of Ag incorporation into ZnO still need to be understood. In addition, as electrochemical doping of ZnO continues to gain interest for cost-effective fabrication, insights into the fundamental mechanisms of this process are especially important. In this work, detailed research has been carried out on the electrochemical doping mechanism of Ag in ZnO using a combination of experimental approaches and theoretical calculations. This unification of experiment and theory is rarely reported in the literature and yet is highly important for a comprehensive exploration and enhanced understanding of the Received: November 8, 2011 Revised: February 22, 2012 Published: February 23, 2012 6383

dx.doi.org/10.1021/jp2107457 | J. Phys. Chem. C 2012, 116, 6383−6391

The Journal of Physical Chemistry C

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2) 32 atom supercell, and the formation energy and electronic properties were calculated on the basis of the optimized supercell.

doping mechanisms. Cyclic voltammetry (CV) of the electrolytes for undoped and Ag-doped nanowire growth was employed to examine the effects of Ag on the electrochemical growth mechanisms. The roles of the applied potential and Ag+ ions in the electrochemical process were established as key elements for producing a ZnO growth environment conducive to p-type doping. The chemical state of the Ag dopants was examined with X-ray photoelectron spectroscopy (XPS) to validate the p-type doping results. Furthermore, these experimental inquiries were combined with calculations using density functional theory (DFT) to achieve a better understanding of the doping process in the nanowires. Both XPS and DFT calculations indicate Ag is in the Zn substitutional site (AgZn) in the doped ZnO nanowires, which shifts the Fermi level toward the valence band and induces p-type conductivity. The complementary results from experiment and theory in this study help to uncover the doping mechanism and help to further develop highly conductive p-type ZnO nanowires for potential optoelectronic applications.

III. RESULTS AND DISCUSSION A. Influence of Ag+ on ZnO Growth. Representative SEM images of the undoped and Ag-doped nanowire arrays obtained by our electrochemical process have been presented elsewhere4,25 and, therefore, will not be repeated here. The samples have typical nanowire diameters of 100−200 nm and lengths ranging from 0.7 to 2.5 μm depending on the growth conditions. Under mild growth conditions, the ZnO nanowire morphologies are not significantly affected by Ag doping, but it is possible to drastically alter them by using higher Ag concentrations and other applied potentials.23 In the electrochemical process used here, the possible reactions occurring that lead to the deposition of ZnO nanowires are listed as follows:3,29−33 NO3− + H2O + 2e− → NO2− + 2OH−

II. EXPERIMENTAL DETAILS The ZnO nanowire arrays were deposited on gold-coated (20− 40 nm, thermal evaporation) silicon substrates using an electrochemical process at 95 °C.4,25 In short, an aqueous solution with equimolar amounts of zinc nitrate and hexamine (8.4 mM) was used for undoped ZnO growth, while for Agdoped samples 0.05, 0.2, or 0.5 mol % of zinc nitrate was replaced by silver nitrate (silver nitrate = 4.2, 16.8, 42 μM). All samples were obtained under potentiostatic conditions by applying a bias of −0.5, −0.7, or −0.9 V to the substrate (vs an Ag/AgCl reference electrode) for 1 h. The current density during growth was sometimes adjusted by modifying the counter electrode (Au wire) surface area. The CV experiments were performed in the same electrochemical cell as the sample depositions, and the same Ag/AgCl reference electrode and Au wire counter electrode were used. All of the experimental conditions for the CV measurements were arranged to reflect the sample growth conditions. Annealing experiments were performed in air at up to 450 °C for 20 min. The samples were characterized structurally by scanning electron microscopy (SEM; JEOL JSM7000F at 15 kV) and X-ray diffraction (XRD; Siemens D5000, Cu Kα, λ = 1.5418 Å). Their composition was analyzed and quantified with energy-dispersive X-ray spectroscopy (EDX) and XPS. Optical properties were measured by PL using the 325 nm line of a He−Cd laser and a Jobin Yvon 320 spectrometer coupled to a CCD camera. The electrical properties were determined by photoelectrochemical cell (PEC) measurements25−27 in either a 0.005 M NaCl (aqueous) or 0.1 M LiClO4 (propylene carbonate) electrolyte with no real change observed between the two electrolyte solutions. The open-circuit voltage between the nanowire arrays and a gold electrode was monitored under both dark and illuminated conditions. A 150 W halogen lamp white light source was used for illumination. The DFT calculations were performed using the generalized gradient approximation with the Perdew−Burke−Ernzerhof exchange correlation functional.28 The cutoff energy for basic functions was 380 eV, and the Monkhorst−Pack grids used were 4 × 4 × 2 for doped structures. The convergence threshold for self-consistent field iteration was set at 10−6 eV, and all of the atomic positions were fully optimized until all components of the residual forces were smaller than 0.05 eV/Å. The doped systems were constructed from a relaxed (2 × 2 ×

(1)

hydrolysis

C6H12N4(hexamine) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ OH−

(2)

Zn 2 + + 2OH− → Zn(OH)2

(3)

Zn(OH)2 → ZnO + H2O

(4)

Zn 2 + + 2OH− → ZnO + H2O

(5)

Many studies of solution-based processes in the literature list eqs 3 and 4 as the eventual pathway for ZnO crystallization, which involves an intermediary Zn(OH)2 phase.3,29,32 However, a recent investigation by McPeak et al. provided strong support for the idea that eq 5 is the route for ZnO formation: a direct crystallization from Zn2+ and OH− ions that does not involve any intermediate hydroxide phase.31 Furthermore, Ag+ ions are also present in the electrolyte, though at an extremely low concentration relative to Zn, which suggests silver hydroxide or silver oxides may also be deposited along with ZnO by similar routes as those in eqs 3−5. XRD patterns of the Ag-doped nanowires did not provide any evidence for significant formation of Ag related materials, however, indicating that the majority of Ag is doped into the ZnO structure.23 Nevertheless, the possibility for segregation of Ag and its oxides, as well as growth conditions to avoid or minimize their formation, is discussed further in the text. On the basis of the reactions listed above, the availability of Zn for ZnO growth should be more consistent and predictable in comparison to O because it comes directly from the source material. The ability of Zn2+ ions to be present at the growing crystal surface is limited by a diffusion mechanism. The availability of O comes from OH− ions which are produced by the reduction of nitrate ions (eq 1) at the electrode surface and by the hydrolysis of hexamine (eq 2) throughout the electrolyte. There is evidence that the release of OH− by hydrolysis of hexamine is well controlled throughout the deposition process by hexamine also acting as a pH buffer for the growth solution.30 Thus, the production of OH− via the hydrolysis of hexamine should be consistent for the different growth conditions used since the hexamine concentration is held constant for all samples and experiments under consideration here. On the other hand, the availability of OH− from the reduction of nitrate ions is dependent on the 6384



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current densities, but the effect is not as strong as the initial addition of 0.05% Ag. The onset of a cathodic current occurs at slightly more positive potentials in the 0.2 and 0.5% Ag electrolytes. This indicates that when Ag+ is at a high enough concentration it not only significantly increases the current density associated with nitrate reduction but also reduces the overpotential necessary for the electrochemical reaction to readily occur. The new activation of a cathodic current because of the presence of Ag+ may result from a separate electrochemical reaction involving Ag species. However, this identification is not conclusive on the basis of the full CV scan (see Figure 1 inset) as there is no distinct change in its shape to signify a new reaction. Therefore, the increased cathodic current is likely still associated with the electrochemical reduction of nitrate which increases the current density across most of the applied potential range for ZnO deposition (∼ −0.6 to −0.9 V vs Ag/AgCl). The specific effects of the Ag concentration and applied potential on the current density are more readily seen in Figure 2. At a lower potential of −0.6 V, the addition of Ag+ to the

applied potential as well as on the presence of cations in solution. It has been well established that the electrochemical reduction of nitrate ions in aqueous solution is catalyzed by the presence of metallic ions34,35 such as Zn2+.29 Without Zn2+ ions present, there is no observable reduction of nitrate under otherwise similar conditions used for the deposition of ZnO.29 Of course, in the case of Zn2+ ions, they are not actual catalysts because they are consumed during the formation of ZnO, but their basic behavior is best described as a catalyst.29 When Ag+ ions are added in the electrolyte, they may play a similar catalyst role as Zn2+ in the nitrate reduction, which thus affects the ZnO growth. This idea is supported by the comparison of CV scans of growth solutions with and without Ag nitrate in Figure 1.

Figure 1. Cyclic voltammetry (CV) scans of the electrolytes used for undoped and Ag-doped ZnO nanowire growth. The mol % of Ag in the growth solution is indicated in the figure. Only the forward scans (0 → −0.95 V) are shown for clarity. The inset shows full CV scans for the 0 and 0.2% Ag solutions. The scans were performed at a rate of 20 mV/s on substrates with a thin layer of ZnO, and only the first scans are shown as no significant change was observed for repeating scans.

In the undoped ZnO growth solution (0% Ag), an obvious cathodic current begins near −0.65 V. This current is associated with the nitrate reduction electrochemical reaction (eq 1).29,36 As the applied potential is increased negatively, the cathodic current increases because of the increased rate of the nitrate reduction reaction. This in turn leads to an increased ZnO nanowire growth rate on the basis of the reactions in eqs 1and 3−5. The electrolyte used in this work is similar to those commonly utilized for the hydrothermal growth of ZnO where an applied potential is not employed.1,2 In the hydrothermal process, the nanowire growth rate is mostly dependent on the growth temperature and precursor concentrations37−39 and in fact does not change significantly with these growth parameters. On the other hand, with the addition of the electrochemical potential, the growth rate becomes strongly dependent on the applied bias used during growth.4 This effect of the applied potential is even more visible when Ag+ is added to the electrolyte. A small addition of 0.05% Ag does not change the current density for potentials up to −0.65 V. However, once the potential is well within the overpotential range for nitrate reduction,29,36 there is a substantial increase in the current density as compared to the undoped ZnO electrolyte. Further increases in the Ag+ concentration to the nanowire growth solution produce similar increases in the

Figure 2. Current density vs Ag concentration in the ZnO nanowire growth solution at different applied potentials. The data are taken from the CV scans in Figure 1. The solid lines are simply guides for the eye.

growth solution produces only a small, mostly linear increase in the current density. As the cathodic (negative) potential is increased, however, a much more substantial effect of the addition of Ag+ to the electrolyte can be seen. At the largest potentials of −0.8 and −0.9 V, the initial 0.05% Ag in the growth solution creates a considerable increase in the current density followed by a slightly tapered increase for larger Ag concentrations. It is clear that the effect of Ag+ on the growth conditions becomes more pronounced at larger negative potentials. The approaching saturation of the current density with increasing Ag concentration is very similar to the Langmuir adsorption behavior observed in the catalytic role of Zn2+ ions in nitrate reduction by Yoshida et al.29 The CV analysis provides strong evidence that Ag+ is analogous to Zn2+ and other metallic cations adsorbing onto the electrode surface and acting as a catalyst for the nitrate reduction electrochemical reaction. In fact, on the basis of the quite significant increase in current density at a very low concentration relative to Zn2+, Ag+ may be a more efficient catalyst in this sense. 6385



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The reduced rate of oxygen reduction was shown to affect the free electron concentration in the ZnO nanowires presumably in part because of the change in the [Zn2+]/[OH−] ratio during the deposition process.44 The lower rate of OH− ion production in electrolytes with larger Cl− concentrations increases the [Zn2+]/[OH−] ratio leading to Zn-rich growth conditions and increased free electron concentrations from native defects such as Zni and VO.44 In another study by Chatman et al. on ZnO films by electrodeposition using zinc nitrate only, their electron concentration was found to decrease at more negative growth potentials.45 The authors attributed this change to the increased formation of compensating acceptor defects such as VZn at the faster ZnO growth rate produced by more cathodic potentials. As a result of these considerations, it is possible that the electrochemical growth parameters such as the applied potential and presence of Ag+ can also modulate the deposition conditions for ZnO in our process favoring either Zn-rich or Orich environments. Certainly, as the deposition proceeds and ZnO is formed, some of the Zn is used up in principle lowering the [Zn2+]/[OH−] ratio. This should be true especially for larger growth rates. At larger negative potentials and with Ag+ present, the ZnO growth rate is the highest leading to the fastest use of the available Zn. The availability of O from the electrochemical reduction of nitrate ions is also highly dependent on the applied potential, and from the CV analysis for Ag-doped growth solutions, it is clear that the presence of Ag+ creates a similar increase in the rate of nitrate reduction. Therefore, as the applied potential is increased negatively and as Ag+ is added to the electrolyte, the production of OH− ions is substantially increased and the [Zn2+]/[OH−] ratio may decrease considerably. These combined effects shift the growth conditions toward an O-rich environment in turn altering the native defect and Ag impurity formation mechanisms in the nanowires. O-rich conditions are expected to minimize the formation of native donor defects (Zni, VO)46,47 while also lowering the formation energy for native acceptor defects (VZn, Oi)46,47 and Ag impurities.48−50 These ideas could help to explain the results in our previous study where Ag-doped nanowires with p-type properties could only be obtained at a potential more negative than −0.65 V.25 Similarly, we observed that even in undoped ZnO nanowires their PEC responses indicated reduced n-type character at more negative growth potentials,25 which fits well with the discussion above and with the work of Chatman et al. on ZnO thin films by electrodeposition.45 B. Composition Analysis. Figure 4 showcases the effects of Ag concentration during growth on the Ag content in the nanowires. We observe an approximately linear increase in the Ag content of the nanowires as the Ag concentration in the electrolyte is increased. The actual Ag content in the doped nanowires is much larger than the Ag concentration in the growth solution. Such an observation indicates that Ag is readily incorporated into the ZnO nanowires even at a very low concentration relative to Zn in the electrolyte. This may be explained by the enhanced catalytic role of Ag+ in the electrochemical growth process. Much like Zn2+, Ag+ is adsorbed onto the electrode (ZnO nanowire) surface where it acts as a catalyst for the reduction of nitrate.29 Because of its presence at the local site of OH− ion production, it is likely readily incorporated into the ZnO structure in a similar manner to Zn2+. The CV analysis suggests that Ag+ is a more efficient catalyst than Zn2+ for nitrate reduction; therefore, it may be

On the basis of the CV data, it is evident that the presence of Ag+ in the growth solution influences the reaction environment and should cause the ZnO nanowire growth rate to increase. The actual effects of the Ag concentration during growth on the rate of ZnO nanowire deposition can be seen in Figure 3. It is

Figure 3. Growth rate of ZnO nanowires as a function of Ag concentration in the growth solution. Growth under three applied potentials was investigated. The nanowire growth rate was determined by SEM cross section measurements.

clear in Figure 3 that higher negative potentials lead to an increased growth rate in both undoped and Ag-doped nanowires as expected on the basis of the CV scans in Figure 1. More importantly, at each potential used, a distinct increase in the growth rate is observed when Ag+ is initially added to the electrolyte, and then for higher Ag concentrations, the growth rate slowly continues to increase. The changes in the ZnO nanowire growth rate related to the presence of Ag+ in the electrolyte correlate well with the CV data. The growth rate data indicate that it is possible the deposition process becomes too fast to maintain uniform nanowire growth at the very large current densities created by high Ag concentrations (≥1% Ag). Similar current density increases have been observed in other work on the electrochemical growth of doped ZnO.11,40 It has also been shown in other doped ZnO by low-temperature solution routes that the typical ZnO nanowire structure is difficult to maintain for higher doping levels.41,42 Most likely, the combination of increased Ag doping and a faster ZnO growth rate at high Ag concentrations facilitates the transformations in the ZnO morphology we have observed previously.23 Consequently, to achieve sufficient Ag doping while also maintaining the desired, high-quality ZnO nanowire structure, the electrochemical growth conditions must be chosen carefully and tuned appropriately. The environment for the production of ZnO with regard to Zn and O may also be dependent on the electrochemical growth conditions. The relative concentration of Zn2+ and OH− ions ([Zn2+]/[OH−] ratio) near the electrode surface is likely a key factor in determining the growth conditions for ZnO in this process. Since [Zn2+] is mostly limited by diffusion of Zn2+ ions, the rate of OH− ion production should be the strongest element in determining the [Zn2+]/[OH−] ratio. Tena-Zaera et al. have shown in the electrodeposition of ZnO nanowires via reduction of dissolved oxygen that increasing Cl− concentrations in the electrolyte actually decrease the rate of oxygen reduction and, therefore, OH− ion production.43 Such an effect is opposite to the catalytic role played by metallic ions such as Zn2+ (ref 29) and Ag+ as observed here in nitrate reduction. 6386



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Figure 4. Ag content measured by EDX in the doped ZnO nanowires as a function of Ag concentration in the growth solution.

Figure 5. X-ray photoelectron spectra of Ag-doped ZnO nanowires: full survey and Zn 2p3/2 region (inset). The peaks are labeled in the figure.

more easily incorporated into ZnO than Zn despite its very low concentration relative to Zn. This may also help to explain the necessity to maintain a very low Ag+ concentration for obtaining high-quality and well-structured Ag-doped ZnO nanowires. Because of the very low concentration of Ag relative to Zn, Ag incorporation is highly limited by diffusion. As a result, the nanowire growth rate also affects the final Ag content because faster ZnO deposition limits the possibility for Ag incorporation.25 This likely explains the trend of lower Ag contents in the samples deposited at more negative potentials but at the same Ag concentration. ZnO doped with other materials by electrochemical methods has shown similar behavior40,51,52 suggesting that the electrochemical doping process in ZnO maintains universal traits. When the combined growth conditions include a high Ag concentration in the electrolyte and a low negative potential (e.g., −0.5 V), the measured Ag content is particularly high. Although we have not found direct evidence (e.g., with XRD data),23 this observation may stem from the formation of separate Ag phases such as hydroxides or oxides. Under such growth conditions, the ZnO deposition rate is much lower and the number of available Ag+ ions is increased allowing for Ag to potentially participate in the chemical reactions (eqs 3−5) more readily. Consequently, lower Ag concentrations and more negative growth potentials are favorable for the deposition of high-quality, Ag-doped ZnO nanowires. A full survey and Zn 2p3/2 XPS spectra of the Ag-doped ZnO nanowires are shown in Figure 5. The C 1s peak at 285 eV was used as a reference for the binding energy scale.53,54 Strong signals from Zn, O, and adventitious C are observed in the full survey spectrum. The presence of carbon is expected from surface contamination since the sample had been exposed to air before the XPS measurements. Weak peaks at about 367 and 374 eV from Ag dopants were also detected helping to further verify the incorporation of Ag into the ZnO nanowires. Two peak contributions were required to fit the Zn 2p3/2 region, one at 1021.6 eV and one at 1022.7 eV. The more intense peak at 1021.6 eV is attributed to ZnO,21,54 while the higher energy peak contribution is likely from Zn(OH)2 on the basis of its actual energy position as well as shift from the ZnO peak.55 The presence of Zn(OH)2, or perhaps Zn−OH bonds on the surface of the ZnO nanowires, is not necessarily unexpected considering their exposure to air, deposition in aqueous solution, and possible formation pathway (eq 3).54

C. Chemical State of Ag in ZnO. In an effort to support our previous p-type doping results as well as to understand the doping mechanism, the possible chemical states of the Ag dopants need to be clarified. To achieve this goal, detailed XPS analysis on Ag 3d has been employed in combination with DFT calculations. Figure 6 shows the XPS spectra of Ag 3d taken from doped ZnO nanowires with different Ag contents as measured by EDX. Two main peaks from Ag at 367.5 and 373.5

Figure 6. XPS spectra of Ag 3d for doped ZnO nanowires with different Ag contents. Fitting curves are also included in the figure. 6387



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eV were observed in the doped nanowires. The intensities of the Ag signals increase along with the Ag content as measured by EDX showcasing the consistency of the two measurements and the Ag-doping in the nanowires. Ag peaks from the more highly doped nanowires can be well fitted with two components at 367.5 and 368.4 eV for 3d5/2 and at 373.5 and 374.4 eV for the 3d3/2 peak. The lower binding energy components in the spectra (∼367.5 and 373.5 eV) are attributed to Ag+ as in AgZn surrounded by oxygen. Previous work on Ag-doped ZnO,20,22,56,57 including p-type Ag-doped ZnO,20 has consistently shown Ag 3d5/2 binding energies near 367.5 eV, 0.5−0.7 eV lower in energy than expectations for metallic Ag.58 Along this line, the higher binding energy contributions are likely from Ag2O or metallic Ag, which have also been observed in Ag-doped ZnO when growth or annealing conditions led to Ag aggregation.17,20,56 Importantly, the lower binding energy components are consistently stronger than the higher binding energy contributions for the samples in Figure 6. This provides strong evidence that AgZn is the major dopant state for Ag in the ZnO nanowires. The chemical states of Ag2O or metallic Ag are also observed because of possible aggregation of Ag in ZnO at increasing doping levels.17,49,56 To provide support for the experimental results, the lattice parameters and formation energies of Ag-doped ZnO with different dopant occupation sites were calculated using DFT. The doped ZnO was constructed with Ag substitution on the Zn site (AgZn, Figure 7a), on the O site (AgO, Figure 7b), and

Table 1. Crystal Parameters for Undoped and Ag-Doped ZnOa lattice parameters (Å) dopant state

a=b

c

undoped (DFT) AgZn (DFT) AgO (DFT) Agi (DFT) undoped (XRD) Ag-doped (XRD)

3.249 3.293 3.313 3.331 3.245 3.262

5.205 5.371 5.427 5.571 5.195 5.229

a

DFT and XRD represent calculated and experimental values, respectively.

tally16−19,21 and theoretically.49 We also observed by XRD both a- and c-axis lattice expansion in our Ag-doped nanowires,23 and the extrapolated lattice parameters are included in Table 1. From the DFT calculations, only the values for AgZn (a = 3.293 Å and c = 5.371 Å) are close to the experimental data obtained from XRD (a = 3.262 Å and c = 5.229 Å). The other two dopant states (AgO and Agi) cause too large of a change in the lattice parameters, which is not observed in the XRD data. The DFT calculations indicate that AgZn is the most likely dopant state in ZnO, which is consistent with the XPS data. To further confirm that AgZn is the most stable dopant in ZnO, the dopant formation energies, Ef, were calculated according to the following formula:49,50 E f = E(doped ZnO) − E(ZnO) + ΔnZnμ Zn + ΔnOμO − ΔnAg μAg

(6)

Here, E(doped ZnO) and E(ZnO) are the total energies of the supercell with and without Ag dopants, respectively, Δn is the number of ions (Zn, O, or Ag) replaced in the system, and μ is the chemical potential of Zn, O, or Ag. The chemical potentials depend on the material composition, that is, Zn-rich or O-rich. For Zn-rich conditions, ZnO is assumed to be in equilibrium with Zn metal, whereas it is in equilibrium with O2 gas for Orich conditions. The reservoir chosen for Ag is Ag2O in equilibrium with Ag metal or O2 gas. The calculated formation energies for Ag dopants in different lattice locations are summarized in Table 2. The formation energy calculations Table 2. Formation Energies Ef of Doped ZnO under Both Zn-Rich and O-Rich Conditions Ef (eV)

Figure 7. The geometries of the models for (a) AgZn defect, (b) AgO defect, (c) Agi defect after optimization by DFT. The Zn, O, and Ag atoms are represented by gray, red, and blue spheres, respectively.

dopant state

Zn-rich

O-rich

AgZn AgO Agi

2.9 2.0 6.0

−2.2 3.8 4.3

showcase that Ag prefers to substitute the Zn site under both Zn-rich and O-rich conditions but especially under O-rich conditions. Ag may also substitute the O site under Zn-rich conditions when Zn and Ag are both sufficient. However, the formation energies of Agi defects in ZnO are quite high in both Zn-rich and O-rich environments indicating that they may hardly form under these conditions. The defect formation energy results suggest that it is possible to create significant AgZn impurities in ZnO while avoiding the occupation of other lattice locations that would cause compensation. Previous

in the interstitial site (Agi, Figure 7c). The calculated lattice parameters of undoped and Ag-doped ZnO are given in Table 1. For undoped ZnO, the calculated lattice parameters are a = 3.249 Å and c = 5.205 Å, which are in very good agreement with the values a = 3.250 Å and c = 5.207 Å for bulk ZnO.59,60 When introducing Ag impurities into ZnO, the size of the unit cell swells in three directions for all three types of dopant coordinations. Many other studies on ZnO have shown lattice constant expansion because of Ag-doping both experimen6388



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theoretical calculations for Ag-doped ZnO have yielded similar conclusions: under most growth conditions, the substitution of Zn is the most stable location for Ag impurities in ZnO, and an O-rich environment provides the optimum pathway to AgZn doping.48−50 Because the XPS data demonstrated that most of the Ag dopants are in the Zn substitutional site, it can be inferred that the ZnO nanowires are deposited under O-rich conditions. As discussed above, the shift toward O-rich conditions may in fact be provided by the presence of Ag+ and its catalytic role in enhancing the nitrate reduction electrochemical reaction. Interestingly, this means that in the electrochemical technique used here the addition of Ag to the growth process actually facilitates its improved doping efficiency into the ZnO structure. D. Ag Induced p-Type Doping. A particular difficulty in the case of ZnO nanowires is their small size and unique geometry limiting available electrical characterization techniques. While it is possible to fabricate and characterize field effect transistors using single nanowires,13,61,62 such a method proves to be complicated especially in the case of solutiongrown nanomaterials. We utilized a more straightforward approach to probe the electrical properties of our ZnO nanowire arrays with a PEC measurement as described in a previous publication.25 While there are limitations of the PEC technique, it provides important information regarding the conductivity type and the p-type capability of the ZnO nanowires. The details of the PEC measurement method can be found in the literature.25−27 Figure 8 shows the PEC signal as a function of Ag content in the nanowires for as-grown samples and for those annealed at 350 and 450 °C. These samples under investigation show positive PEC responses suggesting that p-type conductivity is achieved. Relative to as-grown, annealing at 350 °C leads to much stronger p-type responses in most of the samples, but the overall PEC signal is then reduced after annealing at 450 °C. In general, these lower annealing temperatures produce improved and more consistent positive PEC responses in comparison to higher temperature annealing at 600 °C.25 Although the applied potential does not show a significant impact on the PEC response for as-grown samples, its pronounced effect was observed after a mild annealing at 350 °C. For a given Ag content, an increase in the positive PEC signal is observed as the applied potential is increased negatively. This observation correlates well with the CV and DFT analysis above. For Agdoped ZnO electrolytes, a larger cathodic potential likely helps to produce growth conditions that are favorable for efficient AgZn-doping and optimal native defect contributions leading to enhanced p-type properties. These results indicate that the applied potential plays an important role in determining the electrical properties of the annealed nanowires with similar Ag contents. It has been observed that definitive p-type doping requires the host ZnO material to maintain its high crystalline quality. However, the introduction of dopants into ZnO generally causes the amount of defects to increase, which is not desirable for effective Ag doping. Our PL studies have shown that an increased negative potential during growth helps to reduce the defects induced by the doping process and thus helps to minimize the negative impact on crystal quality because of Ag doping. This is further confirmed by decreased defect PL emissions after annealing at 350 °C and by enhanced PEC responses. The experimental results have shown that Ag dopants can be acceptors in ZnO nanowires and can generate p-type

Figure 8. Photoelectrochemical cell (PEC) data for Ag-doped ZnO nanowires as a function of Ag content (EDX) separated by growth potential: (a) as grown; (b) 350 °C anneal in air; (c) 450 °C anneal in air. All of the data are presented using the same scales in order to more easily compare the effects of annealing and Ag content on the PEC values.

conductivity. XPS measurements also suggest that the majority of Ag dopants are AgZn in the nanowires. These observations are further supported by band structure and density of states (DOS) calculations. Figure 9 shows the band structure of both

Figure 9. Calculated band structures of (a) undoped and (b) Agdoped ZnO with Ag substitution for Zn (AgZn). The dotted line in each figure represents the Fermi level.

undoped and Ag-doped ZnO with Ag at the Zn substitutional site. It is evident that introducing a AgZn dopant shifts the Fermi level downward in ZnO indicating that AgZn behaves as an acceptor, that is, p-type doping. From the partial DOS shown in Figure 10, it can be seen that the newly formed gap states are contributed by hybridizations between Ag 4d and O 6389



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highly conductive p-type materials using electrochemical growth techniques.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 501-569-8962. 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, EPS-1003970. The authors would like to thank the UALR Nanotechnology Center for use of its SEM facilities.

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REFERENCES

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Figure 10. DOS and partial DOS of (a) undoped and Ag-doped ZnO with (b) AgZn, (c) AgO, and (d) Agi defects. The dotted line represents the Fermi level.

2p states. Because of Ag’s relatively large ionic radius and low atomic d orbital energy, the p−d repulsion between Ag 4d and O 2p states is not so strong to push the Ag impurity level too high for p-type doping.48 On the other hand, AgO and Agi defects also induce new partially occupied states inside the band gap but shift the Fermi level upward toward the conduction band yielding n-type properties in ZnO. Prior DFT calculations for Ag-doped ZnO have reached similar conclusions in that AgZn doping can lead to a moderately shallow impurity level above the valence band maximum and can possibly create ptype ZnO.48−50 The DFT calculations are consistent not only with the observed experimental results of the electrical properties of Ag-doped ZnO but also with the structural parameters associated with the Ag-dopant occupation site and stability.

IV. CONCLUSION The Ag doping mechanism in ZnO was explored by a combination of experimental and theoretical techniques. In the electrodeposition process used here, the presence of Ag+ was shown to alter the ZnO growth environment by catalyzing and enhancing the nitrate reduction electrochemical reaction. This effect was found to be similar to an increased negative growth potential leading to a faster ZnO growth rate as well as to a shift toward O-rich growth conditions favorable for Ag doping. XPS and XRD analysis, along with lattice parameter and formation energy calculations by DFT, combine to showcase that Ag substitution for Zn is the most stable Ag impurity in the doped nanowires. Furthermore, band structure and density of states calculations revealed that Ag 4d and O 2p states interact to form an impurity band which shifts the Fermi level toward the valence band maximum and induces p-type properties in ZnO. Electrical characterization of the Ag-doped nanowires illustrated that their p-type properties are dependent not only on Ag content but also on the electrochemical growth conditions and postgrowth annealing. The results obtained in this study help to optimize Ag-doping processes in ZnO for 6390



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Mechanism of Ag Doping in ZnO Nanowires by Electrodeposition

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