Mn3O4 Nanowires

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Hydrothermal Synthesis of Core/Shell ZnO:Mn/Mn3O4 Nanowires Micah Eastman,† Aaron Besser,† Yanxue Chen,‡ and Jun Jiao*,† † ‡

Department of Physics, Portland State University, Post Office Box 751, Portland, Oregon 97207-0751, United States China School of Physics and Microelectronics, Shandong University, Shandong Province, China ABSTRACT: Vertically aligned ZnO nanowires were grown on SnO2:F-coated SiO2 substrates utilizing a low-temperature, seeded chemical bath method. Via additional hydrothermal postprocessing, Mn-doped nanowires were formed with and without a mixed polycrystalline MnO/Mn3O4 shell structure depending upon reagent concentrations. Electron microscopy techniques were implemented to quantify structure, composition, valence state, and morphology. Optoelectronic properties were also measured, including absorption spectra and photoluminescence. These properties were characterized as a function of the duration of both growth and doping time. Our results provide insights about the doping mechanism. ZnO band gaps were found to red-shift and were dependent on both doping time and ZnO growth time. Doping resulted in an increase in lattice spacing as well as an increase in intensity of photoluminescence defect bands.

’ INTRODUCTION Practical applications of ZnO are widely known, and it is one of the more prevalent wide band gap semiconductors being explored in research. It is most known for its potential as an n-type window material in solar cells.1,2 ZnO has been thoroughly explored in many forms: as a thin film, nanowires, nanoparticles, etc.3
5 It has also been demonstrated that Mn-doped ZnO has ferromagnetic properties at room temperature and is therefore of interest in spintronics applications.6
8 In addition, the introduction of such impurities has been shown to red-shift the band gap of ZnO and cause other changes in its optical characteristics, such as its photoluminescence.9 This allows the material to be potentially beneficial for solar cells, as the dopants can cause reduction of near band-edge electron-hole pair recombination as well as internal down-conversion of higher energy photons, which could be absorbed by a narrow band gap material. The ZnO/Mn3O4 core-shell structures produced in this study also show promise for lithium-ion battery applications. The substrate-grown nanowires provide a large functional surface area and the manganese oxide crystallites would be a highly favorable cathode material once enriched with lithium. Based on the length and density of the structures grown in this study, an estimated 20
30-fold increase in surface area would be observed in comparison to a thin film. This would result in a corresponding increase in battery capacity. To explore the potential of this material, optimize synthesis procedures, and develop further insights about the doping mechanism, a study of growth processes and properties was implemented. Reagent concentrations were varied to establish doping and crystallite formation regimes. Upon establishment of optimal ratios, reactions were carried out for various times, and the results are presented herein. ’ EXPERIMENTAL METHODS Synthesis. Synthesis of nanowires was carried out by a seeded

chemical bath method. The seed solution was prepared as 0.750 M r 2011 American Chemical Society

Zn(CH3COO)2 and 0.750 M methenamine in deionized (DI) H2O. To assist in dissolution of the zinc acetate, the solution was heated to 60 °C for 30 min under agitation. SnO2:F-coated SiO2 substrates were cleaned for use by a two-part washing process: a 10 min isopropyl alcohol bath followed by a 10 min DI-H2O bath, both involving sonication. The substrates were allowed to air-dry. The seed was transferred to the substrates via pipet and distributed via spin coating at 1500 rpm for 90 s. To ensure a uniform seed layer, the substrates were then annealed for 1 h at 400 °C. To reduce thermal strain on substrates, the samples were inserted after the furnace was preheated to 100 °C; the furnace was subsequently ramped to the target temperature at an average rate of 20 °C/min. The samples were permitted to cool in the furnace after it was turned off until they had reached room temperature. Growth of ZnO nanowires was carried out by use of a growth solution of 27.8 mM Zn(NO3)2 and 27.8 mM methenamine in DI-H2O. The samples were placed in solution in sealed stainless autoclaves, and the seeded surface was not permitted to face upward, to prevent precipitates from settling on the nanowires. Growth took place in the containers at 95 °C. Reaction times were varied between 8 and 20 h, and variations therein are discussed in the Results section. Following growth, the samples were aggressively rinsed in DI-H2O and were annealed (in a fashion similar to the seed anneal process) with a target temperature of 500 °C. Postprocessing of the nanowires was carried out in one of two solutions: 0.200 mM KOH and 0.400 mM Mn(CH3COO)2 in methanol or 50.0 mM KOH and 16.7 mM Mn(CH3COO)2 in methanol. The former solution was used to produce Mn-doped nanowires, whereas the latter yielded doping as well as surface-bonded manganese oxide crystallites.

Received: January 27, 2011 Revised: April 21, 2011 Published: May 18, 2011 10979

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The Journal of Physical Chemistry C To differentiate between the two processes, the solutions will be henceforth referred to as the “alkali-lean” and “alkali-rich” doping solutions, respectively. The doping process was handled in a manner similar to the growth, albeit at a temperature of 70 °C. Doping times varied between 12 and 48 h. A post-rinse was performed with methanol, and the samples were subsequently annealed at 400 °C according to our previous method. Sample Preparation and Instrumentation. The resulting samples were analyzed via several techniques. An FEI Sirion field emission scanning electron microscope (SEM) was used first to provide morphological information and to provide assurance of consistency of the growth process. In addition, energy-dispersive X-ray spectroscopy (EDS) was performed to detect the presence

Figure 1. SEM micrographs of as-grown (a) 8 h ZnO nanowires at 0° tilt and (b) 20 h ZnO nanowires acquired with 5 kV accelerating voltage.

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of any impurities. Optical absorption curves were then measured on a Shimadzu UV-3600 spectrophotometer and band gaps were extracted via the Tauc relation.10 Photoluminescence (PL) of the samples was carried out with a 337 nm N2 gas laser source (Spectra Physics VSL, 2.4 mW) and an Action Inspectrum 300 spectrometer with a 360 nm cutoff filter. All of the preceding instruments required no sample preparation and were carried out with the asgrown/doped samples. The primary tool used for structural and elemental characterization was an FEI Tecnai F-20 transmission electron microscope (TEM). Crystalline structures and defects were determined by selective area electron diffraction (SAED) and high-resolution (HRTEM) imaging. Elemental and valence-state analyses were carried out on an EDAX EDS system as well as a Gatan parallel electron energy loss spectrometry (EELS) system. TEM samples were prepared by first scraping the nanowires from the substrate into a 5 mL vial. The nanowires were suspended in methanol and sonicated for 20 min. The resulting suspension was pipetted onto a lacey carbon/Formvar Cu TEM grid.

’ RESULTS SEM imaging of as-grown ZnO nanowires shows that they are vertically aligned (Figure 1). In addition, a morphological change from well-defined, hexagonally tipped nanowires toward tapered nanoneedles is observed as the duration of the growth process is increased. This is accompanied by an overall decrease in diameter. Quantitative EDS analysis in the SEM showed minimal change in Zn:O ratio (less than 1% variation).

Figure 2. SEM micrographs of ZnO nanowires treated for 24 h in (a) alkali-lean and (b) alkali-rich doping solutions (images acquired at 30° tilt with 5 kV accelerating voltage). (c) Bright-field TEM image of an alkali-rich processed nanowire showing surface crystallites more clearly.

Figure 3. SAED of ZnO nanowires (a) as grown and (b) doped with alkali-lean solution. (c) SAED of crystallites surrounding a ZnO nanowire treated with alkali-rich solution. 10980

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Figure 4. Intensity profile plot of radially accumulated SAED pattern of surface crystallites produced by treatment with alkali-rich solution. Labeled reflections are due to Mn3O4 except where noted. The unlabeled features are artifacts due to the radial accumulation of the partially masked (000) spot.

Figure 5. HRTEM imaging of (a) as-grown, (b) alkali-lean doped, (c) short-term alkali-rich doped, and (d) long-term alkali-rich doped ZnO nanowires.

After a 24 h treatment with the alkali-lean solution, no morphological change was observed in the nanowires (Figure 2a). However, SEM and TEM imaging of the samples treated in the alkali-rich solution clearly show full encapsulation of the nanowires with surface crystallites (Figures 2b,c). Structural determination of the nanowires was carried out via TEM analysis. SAED patterns from the as-grown and doped ZnO nanowires (Figure 3a,b) show that the wires are of the wurtzite structure. Though the hexagonal faces observed in the initial SEM imaging (Figure 1) suggested that this was the case, the SAED patterns further verify that the nanowires are single crystalline and exhibit c-axis growth. In addition, diffraction analysis shows that the alkali-lean doping solution produced an increase in the ZnO (0001) plane spacing from 5.16 to 5.24 Å (a 1.55% increase). The crystallites produced in the alkali-rich solution were similarly analyzed (Figures 3c and 4). Reflections from tetragonal Mn3O4 (hausmannite) and, to a lesser extent, face-centered cubic (fcc) MnO (manganosite) were observed and are illustrated in Figure 4. Plane spacings were measured, and visible reflections were tabulated, by first performing a rotational

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accumulation about the (000) spot and then drawing an intensity profile across the resulting image. This was carried out with the assistance of the DiffTools plugin for Gatan Digital Micrograph.11 Note that artifacts emerge at lower reciprocal spacing due to the rotational accumulation of the partially masked (000) spot by the diffraction needle; HRTEM imaging was used to supplement the structural information provided by diffraction (Figure 5). As-grown wires, and those doped with the alkali-lean solution, showed no change in surface morphology or crystalline structure (other than the previously measured lattice distortion). Reducing the duration of the alkali-rich doping process permitted viewing of the initial formation of the ZnO/Mn3O4 core/ shell structure (Figure 5c). Such wires exhibited a slightly larger ZnO (0001) plane spacing of 5.32 Å. In addition, the surfacegrown Mn3O4 is clearly visible along with lattice fringes formed from the hausmannite (220) planes, which exhibit 2.03 Å spacing as measured from the image. Elemental analysis of the doped wires is provided in Figure 6. The valence state of the Mn atoms in the wires was determined by quantitative ratios of the Mn L2,3 edges in their EEL spectra. The quantitative method used follows the work of Wang et al.,12 which has produced results in agreement with existing literature. This yielded L2/L3 ratios of 2.83 and 2.38 for the alkali-lean and alkali-rich doping processes, respectively. Quantitative EDS suggests that the Mn dopants are present on the order of 0.40 ( 0.07 at. % and that the Zn:O ratio in the wires is consistent within 0.246% before and after doping with the alkali-lean solution. PL spectra are also provided (Figure 7a) and illustrate the quenching of the near band-edge emission after doping. They also illustrate the increase in green (approximately 560 nm) and wide-band red (approximately 630 nm) emissions, which are characteristic of intrinsic surface oxygen vacancy defects and extrinsic defects, respectively. The observed increases in defect band intensities are discussed later in greater detail as they apply to growth and doping mechanisms as well as band-gap shifts. Band-gap shifts of the doped ZnO structures were measured by optical absorption spectroscopy (applying the Tauc relation) and were found to red-shift relative to undoped ZnO, which was determined to have a band gap of 3.263 eV from its PL spectrum (Figure 7a). A Tauc plot example is provided in Figure 7b, illustrating the relative band-gap shifts of the alkali-lean and alkali-rich treated nanowires (these band gaps were determined to be 3.221 and 3.186 eV, respectively). The magnitude of the band-gap red shift in alkali-lean treated nanowires varied with both initial growth time and duration of the doping process (Figure 7c).

’ DISCUSSION The observed morphology change and average thickness decrease in the 20 h ZnO is likely due to cyclic dissolution and growth of ZnO along its (0001) polar face.13,14 After the Zn salt feedstock is sufficiently consumed, the growth solution reaches an equilibrium of Zn2þ concentration. Material is cyclically etched and redeposited to and from the previously pristine (0001) face, eventually eroding the nanowires and tapering them to a point. As this polar face is consumed, vertical growth rate becomes limited.15 This mechanism allows the nanowires to be altered in overall morphology while still retaining their bulk optoelectronic and structural properties. 10981

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Figure 6. EEL spectra of showing (a) O K and Mn L edges for nanowires with and without crystallites and (b, c) backgroundsubtracted Mn-L edge region of EEL spectra of wires doped with (b) alkali-rich and (c) alkali-lean solutions. Vertical lines denote integration windows used to determine intensity.

The surface-bound manganese oxides are not similarly limited to stacked growth along highly polar planes. Direct polar surface stacking was not observed for Mn3O4 crystallites (i.e., in the [101] direction). This may be due to limited Mn3þ ions in the solution as the Mn3O4 (101) planes contain a higher density of Mn3þ atoms.16 This is consistent with the minor presence of MnO as well as the increased presence of Mn2þ during the alkalilean treatments (as discussed in following paragraphs). HRTEM imaging (Figure 5c) suggests that Mn3O4 growth proceeds in the [220] direction after nucleation begins on the (211) surface (parallel to the ZnO nanowire surface). Compared to the previously mentioned paper,12 the Mn white line intensity ratio of 2.38 measured for the Mn3O4 crystallites is approximately 10% lower than expected and is similarly low compared to the findings of other previous work (2.6
2.8).17,18

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Figure 7. (a) PL spectra illustrating NBE quenching and defect emissions from ZnO treated for 24 h in alkali-lean doping solutions. (b) Tauc plot of UV absorption spectra comparing band gaps of alkalilean and alkali-rich doping solutions. (c) Band gap values for ZnO:Mn nanowires as a function of process time.

The exact intensity ratio, however, has been shown to be rather sensitive to curve-fitting methods.19 The SAED analysis previously established the crystallites as being Mn3O4, which suggests that the fitting methods employed by this study may cause a reduction of this ratio. This, coupled with the nonlinearity of the L2/L3 ratio with respect to valence state, prevents certain quantification of the average valence state measured in the alkalilean samples. The measured ratio of 2.83 definitely suggests a greater concentration of Mn2þ atoms, though it falls short of 4.0, the expected ratio for a pure 2þ state. Furthermore, existing studies that quantify the valence state of Mn dopants in metal oxides have demonstrated the expected ratio for pure Mn2þ dopants.20 We therefore conclude that the alkali-lean samples have dopants in a mixed 2þ/3þ state. 10982

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The Journal of Physical Chemistry C With respect to the dopant concentration, an exact value is difficult to quantify by the methods used, and EDS analysis is not accurate below 1% atomic concentrations. The Mn KR peak appeared with sufficient intensity to consider the peak statistically significant. L-shell peaks were excluded from quantification, and phantom elements, specifically Fe and Cr, whose KR emissions surround those of Mn, were included in quantifications to ensure that zero concentrations were measured for zero-signal elements. As secondary confirmation, trends in band-gap shifts coincide with existing experimental literature for Mn-doped ZnO.21
23 Also of note is the decreased sharpness of the Urbach tail in the Tauc plot of the Mn-doped ZnO, which is typical of Burstein-Moss band-gap shifts.24 The difference in band-gap shifts between 8 and 20 h growth processes as a function of doping time suggests that the doping mechanism is sensitive to the changes in surface chemistry introduced by longer growth times. The band-gap shift of the wires grown for 20 h appears to quickly saturate with respect to growth time. This suggests that the polar (0001) surface is a key factor in the uptake of dopants and that the introduction of Mn dopants is limited by surface bonding with oxygen or direct substitution of Zn atoms upon this crystallographic plane during the hydrothermal treatment. As expected, the 8 h nanowires, which have larger exposed polar surfaces, are able to accept more dopants. These observations coincide with previous findings of similar solution-based methods that have resulted in substitutional Mn dopants.25 Control over the ZnO (0001) surface in this study enabled the identification of this plane as the primary dopant diffusion path. The PL spectra of the nanowires provide further insights concerning potential applications. As expected, a drastic increase in defect band intensity was observed in the doped nanowires. Overlapping the green emission characteristic of surface oxygen vacancies,26 a drastic increase in both shallow and deep trap state emissions (red and orange bands) is observed.27
29 This is accompanied by drastic quenching of the near band-edge emission, indicating a reduced recombination rate for electronhole pairs.

’ CONCLUSIONS Vertically aligned ZnO:Mn nanowires were grown with and without polycrystalline Mn3O4 shell structures, demonstrating that solution-based doping techniques can be easily and reliably applied to substrate-grown structures as well as those suspended in solution. Red-shifted band gaps and lattice expansions were measured as a result of this doping process. Dopant valence states were quantified and their effect on photoluminescence spectra was determined. These measured properties (specifically band-gap shifting, internal down-conversion of UV photons, and decreased NBE electron-hole recombination), make ZnO:Mn an ideal window material for solar cells. In addition, the highly uniform growth and increased surface area lends the ZnO:Mn/Mn3O4 structures to be a promising lithium-ion battery cathode. The demonstrated dependence of dopant uptake upon the polar (0001) ZnO surface is also a useful point that will assist processing optimizations in future applications. Further, these synthesis techniques have been carried out in a repeatable fashion on Si substrates (not presented in this study), which assures that the processes can be easily adapted to other architectures.

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’ AUTHOR INFORMATION Corresponding Author

*Telephone 503-725-4228; fax 503-725-2815; e-mail jiaoj@ pdx.edu. Notes

E-mail [email protected] (M.E.); [email protected] (A.B.); cyx@ sdu.edu.cn (Y.C.)

’ ACKNOWLEDGMENT We thank Debra Gale and Greg Rorrer, Department of Chemistry, Oregon State University, for their assistance with photoluminescence measurements. Microstructural and compositional analysis was performed at the Center for Electron Microscopy and Nanofabrication (CEMN), Portland State University. CEMN is supported by the NSF, Murdock Foundation, ONAMI, FEI Company, Gatan Inc, and PSU. This research was supported in part by the National Science Foundation under Awards CBET 0843179, ECS-0348277, and ECS-0520891. ’ REFERENCES (1) Nadarajah, A.; Word, R. C.; VanSant, K.; K€onenkamp, R. Phys. Status Solidi 2008, 245, 9. (2) Keis, K.; Magnusson, E.; Lindstr€om, H.; Lindquist, S. E.; Hagfeldt, A Solar Energy Mater. Solar Cells 2002, 73, 51. (3) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455. (4) Wan, Q.; Li, Q. H.; Chen, Y. J.; Wang, T. H.; He, X. L.; Li, J. P.; Lin, C. L. Appl. Phys. Lett. 2004, 84, 18. (5) Hoffman, R. L.; Norris, B. J.; Wager, J. F. Appl. Phys. Lett. 2003, 82, 5. (6) Philipose, U.; Nair, S. V.; Trudel, S.; de Souza, C. F.; Aouba, S.; Hill, R. H.; Ruda, H. E. Appl. Phys. Lett. 2006, 88, No. 263101. (7) Ronning, C.; Gao, P. X.; Ding, Y.; Wang, L.; Schwen, D. Appl. Phys. Lett. 2004, 84, 5. (8) Roy, V. A. L.; Djurisic, A. B.; Liu, H.; Zhang, X. X.; Leung, Y. H.; Xie, M. H.; Gao, J.; Lui, H. F.; Surya, C. Appl. Phys. Lett. 2004, 84, 5. (9) Senthilkumaar, S.; Rajendran, K.; Banerjee, S.; Chini, T. K.; Sengodan, V. Mater. Sci. Semicond. Proc. 2008, 11, 6. (10) Tan, S. T.; Chen, B. J.; Sun, X. W.; Fan, W. J.; Kwok, H. S.; Zhang, X. H.; Chua, S. J. J. Appl. Phys. 2005, 98, No. 013505. (11) Mitchell, D. R. G. Microsc. Res. Technol. 2008, 71, 588. (12) Wang, Z. L.; Yin, J. S.; Jiang, Y. D. Micron 2000, 31, 571. (13) Vayssieres, L.; Keis, K.; Hagfeldt, A.; Lindquist, S. E. Chem. Mater. 2001, 13, 12. (14) Kim, G. S.; Ansari, S. G.; Seo, H. K.; Kim, Y. S.; Yang, O. B.; Shin, H. S. Appl. Surf. Sci. 2007, 253, 7197. (15) Guo, M.; Diao, P.; Cai, S. J. Solid State Chem. 2005, 178, 1864. (16) Wang, Z. H.; Geng, D. Y.; Zhang, Y. J.; Zhang, Z. D. J. Cryst. Growth 2008, 310, 4148. (17) Loomer, D. B.; Al, T. A.; Weaver, L.; Cogswell, S. Am. Minerol. 2007, 92, 72. (18) Schmid, H. K.; Mader, W. Micron 2006, 37, 426. (19) Riedl, T.; Gemming, T.; Wetzig, K. Ultramiscroscopy 2006, 106, 284. (20) Zhang, J.; Yu, C.; Liao, Z.; Zhang, X.; You, L.; Yu, D. J. Electron Microsc. 2009, 58, 295. (21) Maiti, U. N.; Ghosh, S.; Nandy, S.; Chattopadhyay, K. K. Physica B 2007, 387, 103. (22) Roth, A. P.; Webb, J. B.; Williams, D. F. Phys. Rev. B 1982, 25, 12. (23) Shinde, V. R.; Gujar, T. P.; Lokhande, C. D.; Mane, R. S.; Han, S. H. Mater. Chem. Phys. 2006, 96, 326. 10983

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