DOI: 10.1021/cg100486g
Surface Dezincification and Selective Oxidation Induced Heterogeneous Semiconductor Nanowire/Nanofilm Network Junctions
2010, Vol. 10 3942–3948
Mehmet F. Sarac, Paresh Shimpi, Julie A. Mackey, Daesoo Kim, and Pu-Xian Gao* Department of Chemical, Materials and Biomolecular Engineering & Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269-3136 Received April 12, 2010; Revised Manuscript Received June 12, 2010
ABSTRACT: By directly heating a surface-dezincified polycrystalline brass (Cu70Zn30 alloy) in ambient conditions, semiconductor nanostructure networks composed of CuO nanofilm and ZnO nanowires have been successfully fabricated. The dezincification induced by metallographic etching on Cu70Zn30 polycrystalline alloy surface created heterogeneous networks composed of individual Cu-rich micrograin surfaces and surrounding grain boundaries. The comparative thermal oxidation investigations show distinct growth of semiconductor nanowire/nanofilm network junctions on dezincified Cu70Zn30 substrates compared to the random ZnO nanowires growth on nonetched brass substrates. The grain boundary region is mostly dominated by ZnO nanowires, while each grain surface was overgrown with CuO nanofilm, forming nanowire/nanofilm network junctions. The different growth habits of ZnO nanowires and CuO nanofilms are due to composition and geometry distribution difference between dezincified Cu70Zn30 grain surface and grain boundary, resulting in heterogeneous surface oxidation kinetics of Cu and Zn element. The surface dezincification and selective oxidation process could provide a new approach for heterogeneous nanostructure design, fabrication, and control. These semiconductor nanowire/film network junctions may be used as nanoscale building block for electronic and optoelectronic devices.
1. Introduction Ordered two- and three-dimensional arrays of uniformly distributed nanostructures are finding potential applications in nanoelectronics, nanooptics, nanocatalysis, bioengineering, etc.1-8 However, no technology is available so far for precisely controlled manufacturing of patterned arrays of two or more dissimilar nanoparticles, nanowires, or nanotubes in an economic throughput. Top-down lithography techniques, such as electron beam lithography, photolithography, and nanosphere lithography have been used to facilitate the self-assembly of position-controlled vertically or laterally aligned nanotube and nanowire arrays.9-13 However, intrinsic disadvantages, such as low throughput, small area, and high equipment cost, have hindered their application. Recently, polymer self-assembly arises as one alternative to uniformly pattern large semiconductor interfaces.14-18 However, the phase separation becomes extremely difficult when the third phase is introduced in blockcopolymers for improving the complexity.19,20 Therefore, search for new techniques with high-throughput, robust, and controlled patterning capability of complex nanostructures remains a significant and challenging task for nanoscience and technology field. Monoclinc CuO and wurtzite ZnO are two of the most abundant and important metal oxide semiconductors on earth. Both of them have been popularly used in electronic and optoelectronic devices including solar cells and chemical sensors. Early reports have demonstrated that heterogeneous polycrystalline microstructures consisting of CuO and ZnO particulate micrograins can be obtained using various preparation methods, such as sputtering,21 sintering,22 and other vapor deposition techniques. It has been reported that CuO/ ZnO heterocontact microstructures fabricated using sol-gel *To whom correspondence should be addressed. E-mail: puxian.gao@ ims.uconn.edu. pubs.acs.org/crystal
Published on Web 07/21/2010
technique has helped increase the H2 sensing capability compared to homogeneous ZnO polycrystals.23 In addition, voltagedependent gas sensitivity24 and CO gas selectivity25 were observed in the CuO/ZnO thin-film heterojunctions. Through introduction of heterocontact or heterojunction between CuO and ZnO, the interplay between the gas species and two sides of the heterostructure23 has added another tuning dimensionality of metal oxide chemical sensors. Specifically, the position and size distributions of heterocontacts and grains play one of the major roles for governing the gas-heterocontact surface interaction induced charge transfer process.23 Therefore, ordered but heterogeneous nanostructures such as CuO/ZnO are desired to further tune and improve the properties and performance of heterostructure-based chemical sensors and other relevant devices. In this paper, we report a simple but new and robust approach for fabricating ordered heterojunction networks of semiconductor ZnO nanowires and CuO nanofilms directly through selective thermal oxidation of surface dezincified Cu70Zn30 polycrystalline substrate. It is suggested that custom design of metallurgically patterned alloy substrates could provide a new approach for fabricating heterogeneous nanostructure assemblies. The demonstrated approach here could open up new possibilities for a variety of complex nanostructure patterning, and heterogeneous nanodevice applications. 2. Experimental Section For the experiments, commercial Cu70Zn30 plate (Busby Metal, 0.25 in. thickness, Cu/Zn atomic ratio of 70/30) was used as the raw material. The Cu70Zn30 plate was machined into a typical dimension of 0.5 in. 0.5 in. before further preparation. To carry out a systematic and comparative study, two types of substrates were prepared: as-polished and surface-etched Cu70Zn30 substrates. The as-polished substrate will go through normal metallographic grinding and polishing process; while the surface-etched substrate will encounter further etching process after polishing. For the metallographic specimen r 2010 American Chemical Society
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Figure 1. (a) a Cu-Zn phase diagram with a Zn content in the range of 0-30 at. %. (b) The fabrication flow process of ZnO-CuO heterojunction nanostructures on Cu70Zn30 polycrystalline substrates. (c) Two types of heterojunctions desired from the fabrication processes shown in b.
Figure 2. Low-magnification SEM images of ZnO nanowires/CuO nanofilms grown on the as-polished Cu70Zn30 substrate at (a) 500 °C for 2 h, (b) 500 °C for 4 h, (c) 600 °C for 2 h, (d) 600 °C for 4 h. The inset in (a) is an SEM image showing the as-polished Cu70Zn30 substrate surface; insets of (b-d) are the zoom-in SEM images. (e) An EDX spectrum shows the composition of the mapped area to be of Zn, O, and Cu in (a). (f) A typical TEM image identifying the growth of ZnO nanowire along [0110], as confirmed by the electron diffraction pattern in the bottom. (g) A typical X-ray diffraction spectrum showing the phase products consisting of CuO, ZnO, and Cu70Zn30 in a typical Cu70Zn30 sample after ambient thermal oxidation at 500 °C for 2 h. preparation, grinding process was proceeded using an increasingly finer grit grinding steps by 80, 180, 300, 600, 800, and 1200 grit SiC papers successively. After grinding process, polishing was carried out using polishing cloths impregnated with a fine alumina powder. For surface-etching of Cu70Zn30 substrates, 90 mL of 0.68 M FeCl3 chemical etchant was prepared in 80 mL of H20 and 10 mL of diluted HCl.
The surface etching was conducted in the etchant solution for 30-120 s to show up phase/grain distribution on the substrate.26 Both types of polycrystalline Cu70Zn30 substrates were heated in a tube furnace at controlled temperatures (∼400-600 °C) for various durations (∼2-4 h) in the presence of air. In the course of experiments, heating rate of brass plate was maintained at 8 °C/min. The as-prepared
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Figure 3. (a) and (b) are a low-magnification and a high-magnification, respectively, SEM images of a metallographically etched Cu70Zn30 polycrystalline substrate showing the GB networks. Inset in (b) is an EDXS composition table revealing the Cu-rich GS after etching process. (c) and top portion of (d) A low-magnification and a high magnification SEM images revealing nanowire bundles preferentially growing along etched GB region after 2 h of ambient oxidation at 500 °C, forming nanowire networks. Inset of (c) is an EDXS composition table revealing the GS and GB composition of the oxidized sample. The bottom portion of d shows two sets of TEM image and selected area electron diffraction pattern identifying the nanowires being wurtzite ZnO growing along [0110] (bottom left, (d), and the nanofilm being monoclinic CuO (bottom right, (d). (e) A typical low-magnification top view SEM image of ZnO nanowires/CuO film heterojunction network after 4 h of ambient oxidation at 500 °C. (f) A zoom-in top view SEM image showing the distinct growth morphology in the GS region (CuO nanofilms) and GB region (ZnO nanowires). and thermally processed substrates were characterized and analyzed using a JEOL 6335F field emission scanning electron microscope (FESEM) equipped with an energy dispersive X-ray spectrometer (EDXS), a Tecnai T12 transmission electron microscope (TEM) equipped with an EDXS, and a BRUKER AXS D5005 X-ray diffractometer (Cu KR radiation, λ = 1.540598 A˚).
3. Results and Discussion Figure 1a is a portion of Cu-Zn binary phase diagram with a Zn-composition range of 0-30 at. % and a temperature range of 0-1200 °C, which suggested a single R-phase component (a face-centered-cubic phase as a Zn-substitutional Cu solid solution) for Cu70Zn30 polycrystalline alloy studied at temperature up to ∼950 °C, its melting point. The fabrication process of ZnO/CuO nanostructure heterojunctions is displayed
in Figure 1b. Two types of Cu70Zn30 polycrystalline alloy substrates have been used in this study. One is as-polished polycrystalline Cu70Zn30 alloy substrate; the other is surface-etched polyctrystalline Cu70Zn30 substrate. It has been found that thermal oxidation products are drastically different on these two substrates. Two types of semiconductor nanostructure heterojunctions are designed to grow after thermal oxidation processes, as illustrated in Figure 1c. One configuration is ZnO nanowires grown on CuO nanofilm, the other is CuO nanofilm surrounded by ZnO nanowires network as defined by grain surface (GS)/grain boundary (GB) networks. Figure 2 is a set of SEM images showing the ZnO nanowires and CuO nanofilms grown on the as-polished Cu70Zn30 substrate after thermal oxidation using four different temperature/ duration processing parameters. The inset in Figure 2a is an
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Figure 4. (a) A typical low-magnification top view SEM image of ZnO nanowires/CuO film heterojunction network after 2 h of ambient oxidation at 600 °C. (b) A zoom-in top view SEM image showing the distinct growth morphology in the GS region (CuO nanofilms) and GB region (ZnO nanowires). (c) The crack started along GBs because of the overgrowth of ZnO nanowires and CuO nanofilm. (d) Typical EDX spectra corresponding to the Zn-rich A (GB) and Cu-rich B (GS) regions, respectively.
SEM image showing smooth and clean as-polished Cu70Zn30 substrate surface; insets of Figure 2b-d are zoom-in SEM images revealing the mixing nature of grown nanowires and nanofilm after thermal oxidation, consistent with the literature report.27 At 500 °C, ZnO nanowires sparsely grew on the as-polished substrate; as the temperature increases to 600 °C, density of nanowires increases drastically. However, as the temperature increased from 500 to 600 °C, shorter and wider nanobelts seemed to populate as revealed in the insets of Figures 2c and 2d. The as-synthesized nanowires are rooted on the substrate with a length of ∼0.5-3 μm and a diameter of ∼40-90 nm. Just like smooth and clean surface revealed in the as-polished substrate, no grain boundaries were resolved after thermal oxidation. The EDXS result shows that nanowires on the oxidized as-polished substrate are ZnO nanowires. Figure 2e displays an EDXS corresponding to the red-rectangle area in Figure 2a, indicating that ∼31 at. % of Cu and ∼38 at. % of Zn is present in the local nanowire regions. The percentage of Zn is higher than that of Cu, suggesting the dominance of ZnO nanowires with the CuO film beneath. Figure 2f displays a typical TEM image of a ∼3 μm long ZnO nanowire growing along [0110], as confirmed by the electron diffraction pattern. The X-ray diffraction pattern (Figure 2g) reveals that thermal oxidation at 500 °C for 2 h led to the formation of monoclinic CuO (JCPDS 45-0937, S.G.:C2/c, a=4.6853 A˚, b=3.4257 A˚, c = 5.1303 A˚, β = 99.549°), wurtzite ZnO (JCPDS 36-1451,
S.G.:P63mc, a = 3.24982 A˚, c = 5.20661 A˚) on Cu70Zn30 (JCPDS 25-0322, S.G.:R3m, a = 4.256 A˚) substrate, S.G., space group.26,27 Figures 3a and b show a low magnification and a high magnification, respectively, SEM images of a 30 s surface-etched Cu70Zn30 polycrystalline substrate. The GB network is clearly revealed with a grain size range of ∼40-100 μm. The EDXS results on etched GS and GB are tabulated in inset of Figure 3b, suggesting both GS and GB were of pure copper after the etching process. After a 2 h ambient oxidation at 500 °C, ZnO nanowires were found to grow on the grain boundaries (Figure 3c and d), with ∼20-30 μm wide single grain as shown in Figure 3c. Nanowire bundles (Figure 3d, top left) clearly distributed along the GB region; while on the GS, some tiny nanorods (Figure 3d, top right) were observed, but no significant nanowires found. In bottom of Figure 3d, TEM characterization results shows typical single crystalline ZnO nanowire with growth direction along [0110] (Figure 3d, bottom left) and CuO film with monoclinic structure(Figure 3d, bottom right). The local composition analysis results were shown in inset table of Figure 3c, corresponding to the rectangle GS area and nanowire bundle. It is clearly shown that Zn, O is dominant in the boundary region, while Cu, O is dominant in the GS. However, the formation of ZnO is dominant becaue of the lower melting point and higher vapor pressure of Zn than those of Cu under the same conditions. In addition, ZnO nanowires
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Figure 5. (a) Zoom-in SEM image showing the crack and peel-off phenomenon on the brass grains upon oxidation on the CuO/ZnO mixture film after 2 h of thermal oxidation at 600 °C. (b) The EDX spectra show the composition of different regions in terms of Zn, O, and Cu.
could grow as a result of a higher outer-diffusion and oxidation rate than Cu, which could penetrate the surface porous Cu and then CuO layer. Therefore, it is clear that CuO film grew on the GS, while ZnO nanowire bundles grew along the GB region, forming ZnO nanowire/film network junctions along the Cu70Zn30 polycrystalline substrate boundaries. With extended oxidation process (4 h), more ZnO nanowire growth has been observed, as shown in Figure 3e and f. Figure 3e clearly shows the GB network represented by the densely packed ZnO nanowire bundles (Figure 3f), with a small amount of nanowires grown on the GS, suggesting the further dominance of Zn diffusion and oxidation on the GS upon prolonged thermal oxidation process. Figure 4a shows a top view SEM image of Cu70Zn30 GS with a certain amount of ZnO nanowire and CuO nanofilm network growth after 2 h of thermal oxidation process at 600 °C. The zoom-in SEM image in Figure 4b clearly revealed the nanowire bundle network surrounding a 20 μm wide grain. The further enlarged SEM image in Figure 4c clearly indicates the crack formation along the GB. Figure 4d further identified the composition of ZnO nanowire bundle on the GB and the CuO based film on the GS.
Figure 5a is a typical SEM image revealing the crack and peel-off phenomenon on CuO/ZnO mixture after 2 h of thermal oxidation at 600 °C. The EDXS result (Figure 5b) shows composition of the three represented region in terms of Zn, O, and Cu: nanowire dominated GB (A), remaining cracked surface layer (B), and exposed surface after peel-off (C). It is clearly seen that remaining surface layer (region B in b) is completely dominated by Cu and O, close to 1:1 by atomic ratio, a further proof of the CuO phase composition. Furthermore, the exposed surface after peel-off has a composition close to original Cu-Zn. It is worth noting that after the peeloff and exposure of mostly nonoxidized Cu-Zn grain surface, GB region is still embedded with a continuous ZnO intermediate layer, as indicated by solid red arrowhead in Figure 5a, as a result of the broken roots of ZnO nanowire bundles in narrowly V-shaped GBs because of surface metallographic etching. It is clear that by using surface metallographic etching, Cu70Zn30 polycrystalline alloy grain boundaries can be exposed in the form of V-shaped trenches across the GB networks, and ZnO nanowires and CuO nanofilm can grow on GB and GS
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which leads to the formation of nanowire/nanofilm network junctions (Figure 6c and d). First, porous Cu-rich GS can easily react with oxygen and form CuO nanofilm on the GS at temperature above 300 °C.30,31 While on the V-shaped GBs, atomic/ionic (O2, Zn, Cu, Zn2þ, Cu2þ) inward and outward diffusions are faster than those across GSs because of the blocking barrier from the Cu-rich GS layer. When copper is oxidized in air at low temperature (
