Langmuir 2006, 22, 10535-10539
10535
Epitaxial Electrodeposition of ZnO on Au(111) from Alkaline Solution: Exploiting Amphoterism in Zn(II)† Steven J. Limmer, Elizabeth A. Kulp, and Jay A. Switzer* Department of Chemistry and Graduate Center for Materials Research, UniVersity of MissourisRolla, Rolla, Missouri 65409-1170 ReceiVed April 29, 2006. In Final Form: July 6, 2006 The amphoteric nature of ZnO is used to produce the material from strongly alkaline solution. The solution pH is lowered globally to produce ZnO powder, and it is lowered locally at a Au(111) surface to produce epitaxial films. ZnO powder is precipitated from a solution of 10 mM Zn(II) in 0.25 M NaOH by simply adding 1 M HNO3 to the solution. For the film electrodeposition, the local pH at the electrode surface is decreased by electrochemically oxidizing the ascorbate dianion. The chemically precipitated ZnO powder grows with a sea urchin-like nanostructure, whereas the electrodeposited films have a columnar structure. ZnO electrodeposited onto a Au(111) single crystal has a ZnO(0001)[101h1]//Au(111)[11h0] orientation relationship.
Introduction Zinc oxide, ZnO, is a transparent n-type semiconductor with a large band gap (3.4 eV) and an excitonic binding energy of 60 meV.1 There is considerable interest in the material because of the possibility of using it to fabricate UV-light-emitting diodes and lasers,2-6 transparent transitors,7 and dye-sensitized solar cells.8-10 It is also of interest for its piezoelectric properties11 and for use as a host semiconductor to dope with magnetic cations to produce ferromagnetic semiconductors with high Curie temperatures for spintronic applications.12-14 The material has been produced by chemical precipitation15-18 and electrodeposition.8,19-34 The ZnO is often deposited in a nanocrystalline columnar form. The general scheme in previous chemical and electrochemical deposition procedures for ZnO is †
Part of the Electrochemistry special issue. * To whom correspondence should be addressed. E-mail: jswitzer@ umr.edu. (1) Ozgur, U.; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogan, S.; Avrutin, V.; Cho, S. J.; Morkoc, H. J. Appl. Phys. 2005, 98, 041301/1041301/103. (2) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897-1899. (3) Wang, X. D.; Summers, C. J.; Wang, Z. L. Nano Lett. 2004, 4, 423-426. (4) Teng, X. M.; Fan, H. T.; Pan, S. S.; Ye, C.; Li, G. H. J. Phys. D: Appl. Phys. 2006, 39, 471-476. (5) Kim, K.-K.; Koguchi, N.; Ok, Y.-W.; Seong, T.-Y.; Park, S.-J. Appl. Phys. Lett. 2004, 84, 3810-3812. (6) Cho, S.; Ma, J.; Kim, Y.; Sun, Y.; Wong, G. K. L.; Ketterson, J. B. Appl. Phys. Lett. 1999, 75, 2761-2763. (7) Fortunato, E. M. C.; Barquinha, P. M. C.; Pimentel, A. C. M. B. G.; Gonc¸ alves, A. M. F.; Marques, A. J. S.; Pereira, L. M. N.; Martins, R. F. P. AdV. Mater. 2005, 17, 590-594. (8) Yoshida, T.; Pauporte, T.; Lincot, D.; Oekermann, T.; Minoura, H. J. Electrochem. Soc. 2003, 150, C608-C615. (9) Yoshida, T.; Iwaya, M.; Ando, H.; Oekermann, T.; Nonomura, K.; Schlettwein, D.; Wo¨hrle, D.; Minoura, H. Chem. Commun. 2004, 400-401. (10) Oekermann, T.; Yoshida, T.; Minoura, H.; Wijayantha, K. G. U.; Peter, L. M. J. Phys. Chem. B 2004, 108, 8364-8370. (11) Ni, H. Q.; Lu, Y. F.; Liu, Z. Y.; Qiu, H.; Wang, W. J.; Ren, Z. M.; Chow, S. K.; Jie, Y. X. Appl. Phys. Lett. 2001, 79, 812-814. (12) Cui, J. B.; Gibson, U. J. Appl. Phys. Lett. 2005, 87, 133108/1-133108/3. (13) Dietl, T.; Ohno, H.; Matsukura, F.; Cibert, J.; Ferrand, D. Science 2000, 287, 1019-1022. (14) Jin, Z.; Fukumura, T.; Kawasaki, M.; Ando, K.; Saito, H.; Sekiguchi, T.; Yoo, Y. Z.; Murakami, M.; Matsumoto, Y.; Hasegawa, T.; Koinuma, H. Appl. Phys. Lett. 2001, 78, 3824-3826. (15) Sounart, T. L.; Liu, J.; Voigt, J. A.; Hsu, J. W. P.; Spoerke, E. D.; Tian, Z.; Jiang, Y. B. AdV. Funct. Mater. 2006, 16, 335-344. (16) Li, Q.; Kumar, V.; Li, Y.; Zhang, H.; Marks, T. J.; Chang, R. P. H. Chem. Mater. 2005, 17, 1001-1006. (17) Tian, Z. R.; Voigt, J. A.; Liu, J.; McKenzie, B.; McDermott, M. J. J. Am. Chem. Soc. 2002, 124, 12954-12955.
to increase the pH of a solution of Zn(II) to induce the precipitation of ZnO. We have previously used the electrochemical generation of base (i.e., pH increase) to electrodeposit epitaxial films of ZnO on single-crystal Au.23 In this article, we demonstrate that the amphoteric nature of ZnO can be exploited to deposit the material from strongly alkaline solution. We show that ZnO powder can be produced from alkaline solution by simply adding acid and that epitaxial films of ZnO can be deposited on Au(111) by electrochemically oxidizing the ascorbate dianion at the Au(111) electrode to lower the local pH at the electrode surface. In both cases, the ZnO is precipitated from strongly alkaline solution by decreasing the pH. Amphoteric metal oxides such as ZnO, Al2O3, SnO2, SnO, PbO, PbO2, and Cr2O3 can act as acids or bases.35 That is, they will dissolve in either acidic or basic solutions. In strongly alkaline solutions, Zn(II) is present as Zn(OH)3- and Zn(OH)42-. If solutions of these anions are acidified at moderate temperatures of about 65 °C, ZnO can be produced by the following acidbase reactions,
Zn(OH)3- + H+ f ZnO + 2H2O
(1)
Zn(OH)42- + 2H+ f ZnO + 3H2O
(2)
Experimental Section ZnO Powder Synthesis. ZnO powder was precipitated from alkaline solution by adding HNO3. The solution was prepared by adding 2.5 mL of a 1 M Zn(NO3)2 stock solution to 250 mL of 0.25 (18) Andeen, D.; Kim, J. H.; Lange, F. F.; Goh, G. K. L.; Tripathy, S. AdV. Funct. Mater. 2006, 16, 799-804. (19) Peulon, S.; Lincot, D. AdV. Mater. 1996, 8, 166-170. (20) Izaki, M.; Omi, T. Appl. Phys. Lett. 1996, 68, 2439-2440. (21) Pauporte, T.; Lincot, D. Appl. Phys. Lett. 1999, 75, 3817-3819. (22) Pauporte, T.; Lincot, D. J. Electrochem. Soc. 2001, 148, C310-C314. (23) Liu, R.; Vertegel, A. A.; Bohannan, E. W.; Sorenson, T. A.; Switzer, J. A. Chem. Mater. 2001, 13, 508-512. (24) Cao, B.; Li, Y.; Duan, G.; Cai, W. Cryst. Growth Des. 2006, 6, 10911095. (25) Chen, Q.-P.; Xue, M.-Z.; Sheng, Q.-R.; Liu, Y.-G.; Ma, Z.-F. Electrochem. Solid-State Lett. 2006, 9, C58-C61. (26) Gao, F.; Naik, S. P.; Sasaki, Y.; Okubo, T. Thin Solid Films 2006, 495, 68-72. (27) Izaki, M.; Watase, S.; Takahashi, H. AdV. Mater. 2003, 15, 2000-2002. (28) Jayakrishnan, R.; Hodes, G. Thin Solid Films 2003, 440, 19-25. (29) Lai, M.; Riley, D. J. Chem. Mater. 2006, 18, 2233-2237. (30) Pauporte, T.; Lincot, D. J. Electroanal. Chem. 2001, 517, 54-62. (31) Xu, L.; Guo, Y.; Liao, Q.; Zhang, J.; Xu, D. J. Phys. Chem. B 2005, 109, 13519-13522.
10.1021/la061185b CCC: $33.50 © 2006 American Chemical Society Published on Web 08/15/2006
10536 Langmuir, Vol. 22, No. 25, 2006 M NaOH that was maintained at 65 °C. If necessary, a few drops of 5 M NaOH was added to produce a clear solution. The ZnO was precipitated by the slow addition of 10 mL increments of 1 M HNO3 until the total volume of added HNO3 was 110 mL. The final pH (measured at room temperature) was 12.7. The temperature of the bath was maintained at 65 ( 2 °C during the addition of HNO3. The ZnO precipitate was collected by filtering, rinsed thoroughly with deionized water, and allowed to dry at room temperature. ZnO Film Electrodeposition. ZnO films were electrodeposited from a solution with final concentrations of 0.1 M NaOH, 10 mM ascorbate (from L-ascorbic acid), and 5 mM Zn(II). A deaerated stock solution of 0.1 M L-ascorbic acid, and 0.05 M Zn(NO3)2 was prepared in Ar-saturated deionized water. The deposition solution was produced by adding 10 mL of this stock solution to an Arsaturated alkaline solution at 65 °C, produced by dissolving 1 g of NaOH in 90 mL of deionized water. The pH of the deposition solution was 13.3. The solution was deaerated to inhibit the facile air oxidation of the ascorbate dianion. ZnO films were electrodeposited using a Brinkmann PGSTAT30 potentiostat at 65 °C under an Ar atmosphere. The deposition was performed at a constant potential of +0.26 V versus Ag/AgCl, yielding a current density of approximately 3 mA/ cm2. The total deposition time was 30 min. The working electrode was a Au(111) single crystal wrapped with a Au wire around the perimeter of the crystal for electrical contact. The Au crystals were electropolished and annealed in a H2 flame. A Pt mesh electrode immersed in the same bath was used as the counter electrode. Deposited films were rinsed thoroughly in deionized water and allowed to dry in air at room temperature. Characterization. The ZnO powders and films were imaged with an Hitachi S-4700 field-emission scanning electron microscope. The powder X-ray diffraction (XRD) pattern was obtained with a Scintag XDS 2000 diffractometer using Cu KR source radiation and a liquidnitrogen-cooled Ge detector. The film XRD scan was obtained with a high-resolution Philips X’Pert diffractometer using Cu KR source radiation with a combination X-ray mirror and two-crystal Ge(220) two-bounce hybrid monochromator (PW3147/00) as the incident beam module and a 0.18° parallel plate collimator (PW3098/18) as the diffracted beam module. Rocking curves were run in a similar configuration but with a triple axis/rocking curve assembly (PW3120/ 60) as the secondary optics. The instrumental broadening is 25 arcseconds. Pole figures were collected on the same instrument in point-focus mode using a crossed-slit collimator (PW3084/62) as the primary optics and a flat graphite monochromator (PW3121/00) as the secondary optics. Cyclic voltammetric (CV) characterization of the electrochemistry of the ascorbate dianion was performed with a Brinkmann PGSTAT30 potentiostat on a polycrystalline Au working electrode with an area of 0.02 cm2. CV scans were run at 50 mV/s in a deaerated solution of 0.25 M NaOH with and without 10 mM ascorbate. The solution was unstirred, and the temperature was maintained at 65 °C.
Results and Discussion Because the deposition of ZnO is performed by changing the solution pH, it is important to know which species are present in solution at various pH values. We have performed our deposition at 65 °C in order to minimize the formation of Zn(II) hydroxides, so the speciation diagram and Zn(II) solubility were calculated at 65 °C. Following the method outlined by Goux et al.,32 we used tabulated values of heat capacity, entropy, and free energy at room temperature to determine the free energy at 65 °C. The standard thermodynamic values were taken from the publication of Goux et al.32 The extrapolated ∆G values were (32) Goux, A.; Pauporte´, T.; Chivot, J.; Lincot, D. Electrochim. Acta 2005, 50, 2239-2248. (33) Pauporte, T.; Cortes, R.; Froment, M.; Beaumont, B.; Lincot, D. Chem. Mater. 2002, 14, 4702-4708. (34) Tan, Y.; Steinmiller, E. M. P.; Choi, K. S. Langmuir 2005, 21, 96189624. (35) Lagowski, J. J.; Sorum, C. H. Introduction to Semimicro QualitatiVe Analysis, 8th ed.; Pearson Prentice Hall: Upper Saddle River, NJ, 2005.
Limmer et al.
Figure 1. Speciation diagram (A) and solubility (B) of Zn(II) as a function of pH at 65 °C. The minimum solubility occurs at pH 10.3. At the deposition pH of 13.3, the solution consists of 77% Zn(OH)42- and 23% Zn(OH)3-.
Figure 2. Scanning electron micrograph image of ZnO powder precipitated by adding HNO3 to a solution of 10 mM Zn(II) in 0.25 M NaOH.
then used to calculate the speciation and solubility diagrams as a function of pH. We have considered Zn2+, Zn(OH)+, Zn(OH)2, Zn(OH)3-, and Zn(OH)42- as the soluble species. ZnO was chosen as the solid phase. The calculated speciation diagram at 65 °C is shown in Figure 1a. Below pH 8, the predominant soluble species is Zn2+. At pH 13.3 used for the electrodeposition of ZnO, the solution consists of 77% Zn(OH)42- and 23% Zn(OH)3-. The solubility of Zn(II) as a function of pH is shown in Figure 1b. The amphoteric nature of ZnO is readily apparent from the Figure. The solubility decreases until a pH of 10.3 is reached. The total Zn(II) solubility at pH 10.3 is 4 × 10-6 M. At higher pH, the solubility increases as a result of the formation of the Zn(OH)3- and Zn(OH)42- species. The concentration of Zn(OH)3reaches a maximum of 93 mol % at a pH of 11.4, after which the concentration of the Zn(OH)42- species begins to increase. At pH 14, the solution consists of 94% Zn(OH)42- and 6% Zn(OH)3-. Figure 1b also demonstrates the two main approaches to producing ZnO in aqueous solution. If the starting solution has a pH of less than 10.3, then the material can be produced by increasing the pH. This is the standard approach that has been
Exploiting Amphoterism in Zn(II)
Figure 3. X-ray diffraction θ-2θ scan of precipitated ZnO powder.
Figure 4. Cyclic voltammograms of 10 mM ascorbate dianion in 0.25 M NaOH + 10 mM L-ascorbic acid (blue, dashed curve) and 0.25 M NaOH (solid, red curve). Both solutions were deaerated and unstirred. The cyclic voltammograms were run at a scan rate of 50 mV/s.
used by us and other groups previously.19-23 If the starting pH is greater than 10.3, then ZnO can be produced by lowering the pH. This is the approach that is introduced in this article. ZnO powder was produced by adding 1 M HNO3 to a solution of 10 mM Zn(II) in 0.25 M NaOH solution. An SEM micrograph
Langmuir, Vol. 22, No. 25, 2006 10537
of the ZnO powders produced by this method is shown in Figure 2. The particles have a pollen- or sea urchin-like morphology. They consist of approximately 200 nm nanorods that are about 1000 nm long that apparently nucleate from a single point and grow radially. The XRD pattern in Figure 3 identifies the material as ZnO, with no evidence of Zn(OH)2. The lattice parameters determined by Rietveld analysis of the entire pattern for the hexagonal material are a ) 0.3240 nm and c ) 0.5188 nm. These compare well with the literature values of a ) 0.32498 nm and c ) 0.52066 nm (JCPDS no. 36-1451). Epitaxial films of ZnO were electrodeposited onto Au(111) single crystals using the oxidation of the ascorbate dianion to lower the pH at the electrode surface. Although the pH could be lowered by oxidizing OH- to O2, the oxidation of ascorbate was chosen for the reaction to lower the pH because the oxidation occurs at a much less positive potential. This should minimize the oxidation of the Au surface and facilitate ZnO epitaxial growth. The electrochemical behavior of the ascorbate dianion on Au was probed by cyclic voltammetry (CV). CVs of 0.25 M NaOH and 0.25 M NaOH plus 10 mM L-ascorbate are shown in Figure 4. The evolution of O2 occurs in the 0.25 M NaOH solution at a potential of about 0.6 V versus Ag/AgCl. The oxidation of ascorbate begins to occur at a potential of about -0.35 V versus Ag/AgCl. The oxidation of either ascorbic acid or the ascorbate dianion is known to produce dehydroascorbic acid.36 As seen in Figure 5, the oxidation of ascorbate will result in the lowering of the pH at the surface. Ascorbic acid is a dibasic acid with pKa1 ) 4.04 and pKa2 ) 11.7.37 At pH below 4.04, the predominant species is ascorbic acid. As shown in Figure 5, the oxidation of ascorbic acid to dehydroascorbic acid involves the loss of two electrons and two protons. At pH above 11.7, the mechanism of acid generation is more complex. The production of dehydroascorbic acid still requires the loss of two electrons, but there are no protons lost. However, it is known that in alkaline solution, dehydroascorbic acid undergoes rapid hydrolysis to produce 2,3diketogulonic acid.38 The production of 2,3-diketogulonic acid would result in a lowering of the pH at the electrode surface. Scanning electron micrographs of a film of ZnO that was electrodeposited onto Au(111) are shown in Figure 6. The ZnO was deposited as a 400-nm-thick columnar film with columns that were about 50-100 nm in diameter and 400 nm in height.
Figure 5. Oxidation schemes for L-ascorbic acid (A) and the L-ascorbate dianion (B).
10538 Langmuir, Vol. 22, No. 25, 2006
Limmer et al.
Figure 8. X-ray rocking curves of ZnO (0002) (A) and Au(111) (B) probing the mosaic spread of the epitaxial film. The fwhm of the ZnO peak is only 0.46°, compared with 0.24° for the Au singlecrystal substrate.
Figure 6. Plan view (top) and cross sectional view (bottom) of an epitaxial film of ZnO on Au(111). The film thickness is approximately 400 nm. The deposition time was 30 min.
Figure 7. X-ray diffraction θ-2θ scan of the epitaxial ZnO(0001) film on the Au(111) single-crystal substrate. The deposition time was 30 min.
The XRD θ-2θ diffraction pattern of the film is shown in Figure 7. In contrast to the powder XRD pattern in Figure 3, which shows all of the expected reflections for ZnO, the XRD of the film on the Au(111) substrate exhibits only the (0002) and (0004) peaks. Notice that the XRD pattern is plotted on a log scale, so minor reflections are at least 104 times less intense than the (0002) peak. These results indicate that the film has a very strong (36) Bartlett, P. N.; Wallace, E. N. K. Phys. Chem. Chem. Phys. 2001, 3, 1491-1496. (37) Lide, D. R. CRC Handbook of Chemistry and Physics, 81st ed.; CRC Press: Boca Raton, FL, 2000. (38) Davies, M. B.; Austin, J.; Partridge, D. A. Vitamin C: Its Chemistry and Biochemistry. Royal Society of Chemistry: Cambridge, England, 1991.
Figure 9. X-ray pole figures of the ZnO film (A) and the Au(111) single-crystal substrate (B). The radial grid lines on the pole figures correspond to 30° increments of the tilt angle, χ. The pole figure shows that the orientation relationship is ZnO(0001)[101h1]//Au(111)[11h0].
(0001) out-of-plane orientation. The out-of-plane orientation relationship with Au(111) can therefore be represented as ZnO(0001)//Au(111). Another indication of epitaxial quality is obtained from X-ray rocking curves. In this experiment, a diffraction angle is chosen for the ZnO(0002) peak, and the sample is rocked back and forth. A perfect single crystal would have a very sharp rocking curve that is broadened only by the divergence of the X-ray beam (25 arcseconds for our instrument). Broadening of the rocking curve peaks is due to mosaic spread in the epitaxial film. The rocking curve for the epitaxial film is compared in Figure 8a to the rocking curve of the Au(111) substrate in Figure 8b. The fwhm values of the ZnO(0002) and Au(111) peaks are 0.46 and 0.24°, respectively. The rocking curves show that the mosaic spread is only slightly larger for the ZnO(0001) film than it is for the Au(111) single-crystal substrate. The in-plane orientation of the ZnO film relative to the Au(111) substrate is determined by X-ray pole figure analysis. Pole figures are acquired by choosing a specific plane to probe while measuring the diffracted intensity as a function of tilt (χ) and rotation (φ). The (101h1) pole figure of the ZnO film is shown in Figure 9a, and the (111) pole figure of Au(111) is shown in Figure 9b. The radial grid lines on the pole figures correspond to 30° increments in χ. The ZnO pole figure exhibits six sharp, equally spaced (∆φ ) 60°) spots at χ ) 61.6°. This agrees with the calculated χ ) 61.6° that corresponds to the angle between the (101h1) and (0001) planes in ZnO. The Au pole figure has a spot at the pole center and three equally spaced spots at χ ) 70.2°, consistent with the [111] orientation. By comparing the two pole figures with the respective calculated stereographic
Exploiting Amphoterism in Zn(II)
projections, the in-plane orientation of ZnO[101h1]//Au[11h0] is determined. The overall orientation relationship showing the outof-plane and in-plane orientation relationships of ZnO relative to Au(111) is ZnO(0001)[101h1]//Au(111)[11h0]. That is, the ZnO(0001) and Au(111) planes are parallel, and the ZnO[101h1] and Au[11h0] in-plane directions are coincident. In this article, we have shown that it is possible to produce epitaxial films of ZnO by electrochemically generating acid at the electrode surface. The method we outline here should also work for the electrodeposition of other amphoteric oxides such
Langmuir, Vol. 22, No. 25, 2006 10539
as Al2O3, SnO, SnO2, PbO, and Cr2O3. Because Al(III) and Cr(III) oxides are amphoteric, the method may also pave the way for the doping of ZnO with Al(III) to increase the conductivity and with Cr(III) to produce a magnetic metal oxide semiconductor. Acknowledgment. This work was supported by National Science Foundation grants CHE-0243424, CHE-0437346, and DMR-0504715andDepartmentofEnergygrantDE-FC07-03ID14509. LA061185B
