Unique Approach toward ZnO Growth with Tunable Properties

Article pubs.acs.org/crystal

Unique Approach toward ZnO Growth with Tunable Properties: Influence of Methanol in an Electrochemical Process Keyue Wu,†,‡ Zhaoqi Sun,†,‡ and Jingbiao Cui*,† †

Department of Physics and Astronomy, University of Arkansas at Little Rock, Little Rock, Arkansas 72204, United States School of Physics and Material Science, Anhui University, Hefei 230039, People’s Republic of China

‡

ABSTRACT: A unique approach has been developed to deposit ZnO with tunable morphology and physical properties by introducing methanol in an electrochemical process. As the methanol increases in the aqueous electrolyte, the growth mode of ZnO transforms from thin films to nanostructures, corresponding to a crystalline orientation change from (002) to (102) and then to (101). These structural changes are accompanied by significant variations of optical and electrical properties, including a widening of the band gap from 3.31 to 3.53 eV, increase of resistivity, and decrease in charge carrier concentration. Compositional analysis indicates that the samples grown at higher methanol concentrations contain Zn(OH)2, which is likely responsible for the band gap change. The growth mechanism is discussed in terms of the impact of methanol on the chemical reaction, i.e., the changing growth mode results from the adsorption of nitrate ions on the polar surface of ZnO. This finding helps to grow specific ZnO nanostructures with desired morphologies and properties for device applications.

1. INTRODUCTION As a wide band gap (3.37 eV) semiconducting metal oxide with a large exciton binding energy (60 meV), ZnO has triggered great interest in the past decade due to its unique properties and potential applications in light emitting diodes, solar cells, gas sensors, and so on.1−4 Recently, growth of ZnO with different morphologies such as nanofilms, nanowalls, and nanowires has been extensively studied.5−7 It was suggested that these structures may have unique physical and chemical properties to satisfy specific device fabrications.8,9 Among the various growth methods, including high temperature vapor processes and low temperature solution routes,10−12 electrochemical growth represents a rapid and cost-effective approach to the fabrication of ZnO nanostructures.13 For example, Inamdar et al. reported that they obtained different ZnO morphologies at different growth temperature.14 Pauporte et al. and Sun et al. investigated the influence of deposition parameters on the ZnO properties and found that the bath temperature, the type of anion, the deposition potential, and the oxygen and zinc ion concentrations could strongly affect the ZnO morphology.15,16 The addition of other chemicals such as KCl, NH4Cl, NH3, H2O2, dimethylsulfoxide (DMSO), and polyvinylpyrrolidone (PVP) into the electrolyte was also investigated in order to change the structure of ZnO.17−21 Although ZnO nanostructures have been investigated extensively, growth of the material with controllable morphologies and properties is still far from satisfactory. In order to meet the requirements of device fabrication, it is critical to synthesize ZnO with desired structures and properties in a controllable way. In this study, the addition of methanol into the electrolyte for ZnO growth is studied for the first time. This represents a new economic approach to tune ZnO’s growth and © 2012 American Chemical Society

properties in an electrochemical process. It was found that the methanol has a significant impact on the growth mode of ZnO with distinct structures and optical and electrical properties that were systematically analyzed by various techniques. The growth mechanism is discussed in terms of the influence of methanol on the chemical reaction near the growing surface of ZnO.

2. EXPERIMENTAL SECTION ZnO nanostructures were deposited in an aqueous solution with the addition of methanol by a low-temperature electrochemical process. The amount of methanol was used to tune the growth mode of ZnO. Zinc nitrate hydrate (Zn(NO3)2·6H2O, 0.025 M) was dissolved in 200 mL of deionized water and methanol at room temperature and then heated to 70 °C on a hot plate. A gold wire with a diameter of 0.2 mm was connected to a gold covered plastic substrate and used as a working electrode. Another gold wire was immersed in the solution and used as a counter electrode. A negative DC potential of −2.5 V was applied to the substrates, which is equivalent to −0.75 V relative to an Ag/AgCl reference electrode. ZnO nanostructures were grown with different concentrations of methanol from 0% to 100% (volume ratio), while other growth conditions were kept constant. At the end of the growth, the sample was removed from the solution and immediately rinsed in flowing deionized water to remove any residual salt from the surface. The morphology of the as-grown nanostructures was characterized by scanning electron microscopy (SEM). The composition of the nanostructures was analyzed with energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). The crystal structure was characterized by X-ray diffraction (XRD) using Cu Kα radiation (1.542 Å). The PL measurements were performed using the Received: January 13, 2012 Revised: March 27, 2012 Published: April 11, 2012 2864

dx.doi.org/10.1021/cg300057s | Cryst. Growth Des. 2012, 12, 2864−2871

Crystal Growth & Design

Article

Figure 1. SEM images of ZnO nanostructures grown with different methanol concentrations in the growth solution: (a) 0%, (b) 30%, (c) 50%, (d) 70%, (e) 90%, and (f) 100%. 325 nm line of a He−Cd laser at constant power and a Jobin Yvon 320 spectrometer with a charge coupled device (CCD) camera. The electrical properties were investigated by resistivity measurements in the van der Pauw configuration as well as Mott−Schottky techniques in solution. The as-grown films on the Au coated plastic were directly used for Mott−Schottky measurements since the Au film is not exposed to the test solution. However, these samples are not suitable for van der Pauw measurements due to the conducting Au beneath the ZnO film. Prior to the resistivity measurements, the ZnO films were transferred from the Au coated plastic to glass substrates by a new technique developed in this lab. Further details of the thin film transfer process are to be published elsewhere; however, similar methods have been described previously in the literature.22 The transfer process does not cause any visible structural damage to the films, indicating that their measured electrical properties should be representative of their true values.

3. RESULTS 3.1. Morphology and Structure Analysis. Figure 1 shows the SEM images of ZnO nanostructures grown for 30 min with different methanol concentrations in the electrolyte. The surface morphologies were significantly changed as the methanol concentration was increased in the growth solution. For the 0% methanol solution, hexagonal and triangular structures were observed as shown in Figure 1a. When the methanol concentration was increased to 30%, the ZnO film still shows a similar surface morphology to that grown in aqueous solution. The average crystal size increases as shown in Figure 1b. This may indicate an increase in the growth rate as methanol was added. Upon further increase of the methanol concentration to 50% (Figure 1c) and 70% (Figure 1d), the crystal morphology changed to a ridge-like shape with very 2865



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30%) exhibit very strong (002) diffraction peaks along with a few other weak peaks, indicating that the films grew along the caxis. For the sample grown at 50% methanol, the intensity of the (002) diffraction peak decreases, while the (102) diffraction peak becomes the strongest one. This suggests that the ZnO growth direction switches from (002) to (102) orientation. With a continued increase of methanol to 70%, the (101) diffraction peak dominates the XRD pattern, suggesting that the growth direction is along (101) now. For the sample grown at 90% methanol, the (101) peak stays as the strongest one among the ZnO diffractions. Note that the overall intensity of ZnO is weak, and diffraction peaks from Au become relatively strong because more Au is exposed to the XRD measurement in the nanosheet structure. In addition to the expected peaks from ZnO and Au, the sample grown at 90% methanol shows a new peak around 33°. Pradhan et al.24 observed a similar peak from their Cl assisted growth and attributed the new peak to the hydroxyl compound Zn5(OH)8Cl2·H2O. However, our samples were grown without inclusion of Cl in the source materials, and the existence of Cl in our samples was also ruled out by XPS measurements. Therefore, it is unlikely the new peak is associated with the hydroxyl compound. Recently, Gao et al.25 also suggested that the peak is possibly due to a new unknown ZnO phase. It is hard to obtain any ZnO diffraction peak from the sample grown with 100% methanol. This may be due to very thin nanowalls comprising tiny nanocrystals as shown in the Figure 1f. The XRD analysis agrees well with the observed morphology evolution evidenced by SEM. Both measurements indicate that the growth orientation gradually transforms from (002) to (102) and then to (101) as the methanol concentration increases in the growth solution. The lattice constants calculated from XRD peaks, ranging from 3.2450 Å to 3.2550 Å and from 5.2156 Å to 5.2171 Å, respectively, for the a- and caxes, are listed in Table 1. The observed lattice constants are in agreement with bulk ZnO.26 3.2. Composition Analysis. Both EDX and XPS have been used to study the composition of the ZnO nanostructures. Figure 3a shows the EDX spectra taken on samples grown at different methanol concentrations. The oxygen intensities have been normalized so that the relative intensities of Zn are comparable. One can see that the Zn intensities decrease at higher methanol concentrations. The actual Zn/O atomic ratio versus the methanol concentration is shown in Figure 3b. It should be pointed out that the Zn/O atomic ratio for the samples grown with methanol concentration higher than 90% is not accurate due to the influence of substrate. ZnO nanosheet structures are vertically aligned on the substrate at higher methanol concentrations. During EDX measurements, the signal from the plastic substrate was therefore also detected in such samples and the Zn/O atomic ratio becomes incorrect. One can see that the relative zinc in the samples continuously decreases as methanol concentration is increased in the growth solution. The low zinc in the nanowall and nanodisk samples could be due to higher Zn(OH)2 mole percentages in these nanostructures as confirmed by XPS measurements. Figure 4 shows the typical XPS spectra of ZnO films grown in pure water and pure methanol. The Zn 2P1/2 (1021 eV), Zn 2P3/2 (1044 eV), O 1s (530 eV), and C 1s (285 eV) were observed. The binding energies are calibrated by taking the carbon C 1s peak as a reference. The sample grown with pure methanol has a relatively stronger O 1s peak, indicating a

narrow top surfaces. Very thin nanosheets grown vertically on the substrate were obtained when methanol was increased to 90% as shown in Figure 1e. Figure 1f shows the top view of ZnO grown in the 100% methanol solution. A nanowall structure with height of 2−3 μm and thickness of 50 nm was observed. Interestingly, the nanowalls were made up of lots of nanodots as shown in the inset of Figure 1f. As is often found in the literature, it was difficult to prepare samples with a consistent morphology and homogeneity when utilizing a water only solution.23 Voids about a micrometer in size were observed over the whole surface of the sample grown in the pure aqueous solution as shown in Figure 1a. The density of the voids is up to 5 × 108 per cm2. However, the ZnO film grown with the addition of methanol in the solution is very uniform and free of voids. Recently, Musselman et al. also showcased an improvement in the large scale uniformity of ZnO thin films by adding 25% ethanol to a typical zinc nitrate deposition solution.23 It is obvious that the growth mode of ZnO is strongly dependent on the methanol concentration in the electrolyte. As the methanol concentration increases, the growth direction transforms from (002) to (102) and then to (101) as evidenced by XRD measurements. In general, the fast growth along the (002) direction tends to form column structures in water solutions, i.e., hexagonal nanowires or nanorods. Therefore, the voids or holes are easily formed. When the growth along other directions dominates, a dense uniform film is easy to form. XRD patterns of the ZnO nanostructures prepared under different methanol concentrations are shown in Figure 2. The

Figure 2. XRD patterns of the ZnO nanostructures grown at different methanol concentrations. The features marked by asterisks are peaks from gold.

diffraction peaks can be indexed to the hexagonal wurtzite structures of ZnO. Note that the diffraction signals from Au film on the substrates were also observed. One can see that the crystal orientation of the films change as the methanol concentration is increased in the growth solution. The samples grown with the lower methanol concentrations (lower than 2866



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Table 1. Lattice Parameters, Band Gap, Carrier Concentration, and Resistivity for ZnO Nanostructures Grown at Different Methanol Concentrations lattice parameters (Å) methanol concentration

a=b

c

band gap, Eg (eV)

carrier concentration (cm−3)

resistivity (Ω·cm)

0% 30% 50% 70% 90% 100%

3.2451 3.2525 3.2549 3.2444 3.2450

5.2171 5.2156 5.2156 5.2171 5.2162

3.30 3.407 3.437 3.451 3.534 3.537

5.137 × 1017 3.186 × 1017 2.25 × 1017 2.00 × 1017

1176 3689 5280 5400 23800

Figure 4. Typical XPS spectra taken on ZnO nanostructures grown in water and methanol.

concentration indicates that the methanol in the growth solution favors the formation of OH in the films.4 Figure 6 compares the Zn 2P1/2 XPS spectra of ZnO grown with different methanol concentrations. The Zn 2P1/2 was at 1021 eV in the samples grown in lower methanol, which indicated the presence of a single Zn2+ divalent state, corresponding to ZnO. However, another peak at 1022 eV evolves as the methanol concentration increases in the electrolytes. This XPS feature follows the spectral evolution of O 1s as shown in Figure 5. The peak at a high binding energy is due to the formation of Zn(OH)2.24 A lower energy shoulder also appears at very high methanol concentrations. The XPS data confirmed that the mole percentage of Zn(OH)2 increases as the methanol concentration increases in the electrolyte. 3.3. Optical Properties. Figure 7a shows the PL spectra of the ZnO samples. A UV peak at about 360−370 nm from the near band edge emission (NBE) and a broad visible peak around 500−750 nm from the defect emission were observed in each sample. The peak position of the NBE emission shifts to a short wavelength as methanol was increased, indicating the increase of the band gap of ZnO samples. Note that there were oscillations in the visible region, which are related to interference in the films. These interference fringes indicate that the thickness of the films increases with the increasing methanol concentration up to 70% in the electrolyte. The band gap energy as a function of methanol concentration is shown in Figure 7b. The measured band gaps of ZnO samples continuously increase from 3.30 eV for the sample grown at 0% methanol to 3.56 eV for the one grown at 100%. The band gap enlargement may be caused by the presence of a small fraction of hydroxide in the films.20 Recently, Pradhan et al. reported that a ZnO film grown at 22 °C has larger optical band gaps up to 4.1 eV. They suggested

Figure 3. (a) EDX spectra of ZnO nanostructures and (b) Zn/O ratio as a function of methanol concentration in the growth solution.

decreased Zn/O ratio. This observation is consistent with EDX measurements. Figure 5a shows the details of O 1s spectra from the samples grown at different methanol concentrations. The O 1s peak obviously contains at least two components. The shoulder at a higher binding energy becomes relatively stronger as the methanol concentration is increased. The O 1s signals can be well fitted with two peaks by a Gaussian function as shown in Figure 5b. The lower energy peak located at 529.97 eV corresponds to O−Zn bonding,27 while the higher energy peak located at 531.25 eV is assigned to the OH species.28 The increase of the relative intensity of OH at a higher methanol 2867



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Figure 7. (a) Photoluminescence spectra of ZnO nanostructures and (b) the corresponding band gap versus methanol concentration. Figure 5. (a) O 1s XPS spectra of ZnO grown with different methanol concentrations. (b) O 1s XPS spectra of ZnO grown in pure water and pure methanol, respectively. The black solid curve is the experimental data, the red curves are cumulative fits to the experimental data, and the green curves are individual peaks from the fitting.

measurements. It is likely that Zn(OH)2 is responsible for the band gap change in the ZnO samples. Although the charge carriers may cause the band gap to change due to the band filling effect or Burstein−Moss effect,30 it is unlikely that this happens in our samples because the carrier concentration of 1017 cm−3 is too low to observe a significant impact on the band gap. It is worth noting that the PL spectra of samples grown with high concentrations of methanol (90% and 100%) are very different from the others. Their UV emissions are very weak while the visible emissions shift to short wavelength. The shift of visible emission is due to the change in defect type and concentration in the ZnO films grown at different methanol concentrations in the solution. The relatively weak UV emissions are associated with the crystalline quality of the films as evidenced by both SEM and XRD measurements. 3.4. Electrical Properties. The charge carrier concentration was determined using electrochemical impedance spectroscopy (EIS) in a propylene carbonate electrolyte (0.1 M LiClO4) to avoid ZnO decomposition. Figure 8 shows Mott−Schottky plots of the samples grown with methanol concentrations from 0% to 70%. Note that it is very hard to measure the Mott−Schottky plot for the samples grown at methanol concentration higher than 70% due to their rough surfaces. A positive or negative slope in a Mott−Schottky plot corresponds to an n- or p-type semiconductor,31 respectively.

that the large band gap is due to the presence of Zn(OH)2 in the samples.29 The presence of Zn(OH)2 in our ZnO samples with large band gaps was indeed confirmed by XPS

Figure 6. Zn 2P1/2 XPS spectra of ZnO grown with different methanol concentrations. 2868



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The hydroxide ions (OH−), which act as the precursors for the formation of zinc hydroxide, can be electrochemically obtained via 2e− + NO−3 + H 2O → NO−2 + 2OH−

(4)

These reactions can be significantly affected by changing the chemical environments in the electrolyte. Previous studies showed that the adsorption of the highly electronegative Cl− ions on the polar (0001) plane could hinder the growth in this direction when KCl was included in the electrolyte.34 The methanol induced change in ZnO growth mode is believed to be associated also with the chemical reaction environmental change near the growing surface. In our XPS measurements, N 1s at about 400 eV was observed in the samples grown with methanol concentrations higher than 70% as shown in Figure 9a. The intensity of the N peak increases with the increase of methanol concentration. Figure 9b shows the details of the N 1s peak taken on the sample grown under 100% methanol. The experimental data can be fitted with three peaks using a Gaussian function. The peak with a binding energy of 400.89 eV corresponds to the core level of N in the form of N−H.35

Figure 8. (a) Mott−Schottky plots of ZnO nanostructures and (b) variation of carrier concentration and resistivity versus methanol concentration.

All the samples under investigation exhibited positive slopes, i.e., n-type conductivity. The donor density, ND, can be calculated from the Mott−Schottky plots32 ⎛ 2 ⎞⎛ d(1/C 2) ⎞−1 ND = −⎜ ⎟⎜ ⎟ ⎝ eε0ε ⎠⎝ dV ⎠

(1)

where C is the depletion-layer capacitance per unit surface area, ND is the donor density, ε0 is the permittivity of vacuum, and ε is the dielectric constant of the semiconductor. The donor concentration and resistivity of the films are plotted in Figure 8b. The resistivity of the samples was measured by the van der Pauw technique. As methanol increases in the growth solution, the donor concentration decreases while the resistivity increases.

4. DISCUSSION In the electrochemical deposition of ZnO using Zn(NO3)2 electrolyte, the reaction involves the formation of hydroxide ions at the surface of the working electrode. These ions react with the zinc ions to form zinc oxide by means of the dehydration of zinc hydroxide. The general reaction can be written as33 2+

−

+ 2OH → Zn(OH)2

(2)

Zn(OH)2 → ZnO ↓ + H 2O

(3)

Zn

Figure 9. XPS spectra of ZnO grown at different methanol concentrations around N 1s peak (400 eV) (a) and details of N 1s peak of the sample grown at 100% methanol (b). The black curve is experimental data, the red one is the cumulative fit, and the dotted blue curves are the individual peaks. 2869



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The peak with a binding energy of 403.82 eV is typically found from oxygen rich nitrogen compounds such as nitrite ions NO2−.36 The one with a binding energy of 406.45 eV is assigned to nitrate ions (NO3−).37 This finding indicates that both nitrite and nitrate ions are deposited onto ZnO structures at high concentrations of methanol. Figure 10 shows a schematic diagram of the morphology evolution mechanism in the presence of methanol. It is well-

Figure 11. Effect of methanol on the chronoamperograms during ZnO nanostructure deposition.

ZnO. It is obvious that the nucleation processes were affected by introducing methanol into the electrolyte. As the methanol concentration increases, the nucleation period becomes short. The nucleation peak disappears when the methanol concentration is higher than 90%, which results in a lesser density of nucleation sites.20 These current measurements show that the concentration of methanol in the solution significantly affects the electrochemical reaction kinetics and results in the different growth mode of ZnO films.

5. CONCLUSIONS ZnO nanostructured thin films with various morphologies were obtained by an addition of methanol in the electrolyte in an electrochemical process. The morphology change from rod-like structures to nanosheets and nanodots resulted from the growth direction change due to the presence of methanol. Both the optical and electrical properties were also significantly modified. The band gap of ZnO was increased from 3.31 to 3.53 eV as the methanol concentration was increased from 0% to 100%. The amount of methanol in the growth solution plays a crucial role in controlling the evolution of the novel ZnO nanostructures. XPS spectra studies reveal that ZnO nanostructures contain Zn(OH)2, which influences the optical and electrical properties of ZnO nanostructures. In addition, a significant amount of N was observed in the ZnO films grown at methanol concentrations higher than 70%, which help to understand the growth mechanism. A growth model that involves adsorption of NO3− on the ZnO polar (0001) face, which hinders the crystal growth along the c-axis and redirects the growth along the nonpolar planes, is proposed.

Figure 10. Schematic diagram of morphology evolutions from hexagonal nanorods to nanodots due to the increased adsorption of NO3− ions onto the growing surfaces in the presence of methanol.

known that ZnO has a wurtzite structure that consists of a polar ±(0001) plane and nonpolar planes such as (101̅0) and (011̅0). The negative ions in the solution are likely adsorbed on the polar plane ±(0001) by electrostatic force, which allows the anisotropic growth of the crystal along the ⟨0001⟩ direction. During the growth, Zn(OH)42− ions, the growing unit in the solution, are adsorbed on the polar face of the ±(0001) surface, resulting in faster growth along the ⟨0001⟩ direction as illustrated in Figure 10a. The addition of methanol into the growth solution may cause the highly electronegative NO3− ions to adsorb preferentially on the polar ±(0001) surface, which hinders the crystal growth along the c-axis and redirects the growth direction along the nonpolar planes as shown in Figure 10b. As a result, the morphology and the crystal orientation were significantly changed as have been discussed in Figures 1 and 2. At very high methanol concentration, the adsorbed NO3− ions on the ZnO nanocrystals would make them hard to continue to grow. Instead, new ZnO seeds are formed and grow into the nanostructures as shown in Figure 10c as well as the SEM image in Figure 1f. The proposed growth model is further supported by the current density change during the electrochemical growth as shown in Figure 11. In all cases, the initial current decreases quickly, which is due to the induction process (the charge− discharge process of double layer) at the interface of substrate/ electrolyte.38 At methanol concentrations lower than 50%, the current increases initially to a maximum value and then decreases. This process corresponds to the initial nucleation of

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AUTHOR INFORMATION

Corresponding Author

*Tel: (501) 569-8962. Fax: (501) 569-3314. E-mail: jxcui@ ualr.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSF (EPS-1003970) and NASA (RID 12008 and 11101). The help for substrate transfer from M. Allan Thomas is greatly appreciated. K.W. and Z.S. gratefully acknowledge the scholarship support from the Chinese Scholarship Council. 2870



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(37) Torres, J.; Perry, C. C.; Bransfield, S. J.; Fairbrother, A. H. J. Phys. Chem. B 2003, 107, 5558−5567. (38) Liu, Z. F.; Jin, Z. G.; Qiu, J. J.; Liu, X. X.; Wu, W. B.; Li, W. Semicond. Sci. Technol. 2006, 21, 60−66.

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Unique Approach toward ZnO Growth with Tunable Properties

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