Shell Nanowires: Synthesis

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Novel Type-II Zn3P2/ZnO Core/Shell Nanowires: Synthesis, Characteristic, and Photoluminescence Properties Peicai Wu, Tuo Sun, Yu Dai, Yanghui Sun, Yu Ye, and Lun Dai* State Key Lab for Mesoscopic Physics and School of Physics, Peking University, Beijing 100871, China

bS Supporting Information ABSTRACT: Novel type-II Zn3P2/ZnO core/shell nanowires were synthesized for the first time. First, Zn3P2 nanowires with high yield were synthesized via a catalyst-free chemical vapor deposition method. Then, a facile low-cost surface oxidization process was used to obtain the ZnO nanocrystalline shells. Field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and highangle angular darkfield scanning transmission electron microscopy (HAADF-STEM) were used to characterize the morphologies, crystal structures, and element composition of the as-fabricated core/shell nanowires. Room-temperature photoluminescence properties of them were studied. A possible growth mechanism was proposed. The heterostructure fabrication approach developed here may be extended to other functional heterostructure material systems. The experimental results together with the energy band analysis demonstrated that the type-II Zn3P2/ZnO core/shell nanowire heterostructures have substantial potential for future photovoltaic nanodevice applications.

A

s an important class of nanoscale building blocks, coaxial core/shell nanowires (NWs) have substantial potential for novel nanodevice applications.1
10 A coaxial heterostructure NW enables carrier extraction across the radius of the NW, while it permits high optical absorption and large current injection along the axial length of the NW.4,8,11 Physical properties of a semiconductor heterostructure primarily depend on the relative alignment of the conduction and valence bandedges of the materials involved. According to the band alignment, heterostructures are typically classified into two types: type-I and type-II. For a type-I heterostructure, the conduction band minimum (CBM) and valence band maximum (VBM) of the semiconductor with narrower bandgap are placed in between those of the other, and both the electrons and holes mainly reside in the narrower bandgap semiconductor. Type-I heterostructures have the advantage in making light emitting devices where higher luminescence efficiency is desirable.12
14 For type-II heterostructures, both the VBM and CBM in one semiconductor are lower in energy than their counterparts in the other, and the electrons and holes are spatially separated. In other words, the electrons will reside mainly in one semiconductor while the holes are in the other. This will decrease the recombination rate of electrons and holes and increase the minority carrier lifetime.12
15 This property is advantageous for photovoltaic device applications. To date, various methods, such as metal organic chemical vapor deposition (MOCVD),1,4,6
8 metal organic vapor phase epitaxy,11 electrochemical deposition,2,13 atomic layer deposition,9 pulsed laser deposition,5 molecular beam epitaxy,16 etc., have been employed to synthesize core/shell and core/multishell NW heterostructures. These approaches open new opportunities of incorporating r 2011 American Chemical Society

multifunctions into an individual NW, which may find superior or new applications in various areas such as solar cells,8,16,17 light emitting diodes,4,18 lasers,11 field-effect transistors,6,7 memories,19,20 thermoelectric devices,21 photoelectrochemical cells,9 and so forth. The absorption spectrum of zinc phosphide (Zn3P2) with a direct bandgap of 1.4
1.6 eV covers the main part of the solar spectrum. Besides this, Zn3P2 has many other advantageous physical properties in serving as a photovolatic material, such as a large optical absorption coefficient (>104 cm
1), long minority carrier diffusion length (∼13 μm), abundant and cheap constituent materials, etc.22,23 Zinc oxide (ZnO), with a wide direct bandgap of 3.2
3.4 eV, is an important optoelectronic semiconductor material. The Zn3P2 and ZnO can form type-II heterostructure,23 which can be used in photovoltaic device applications. Moreover, the wide bandgap ZnO, with small light absorption for solar spectrum, can serve as a window for the Zn3P2/ZnO heterostructure photovoltaic device, which is desirable for obtaining higher conversion efficiency. In this work, we report for the first time the synthesis of novel Zn3P2/ZnO core/shell heterostructure NWs by combining a catalyst-free chemical vapor deposition (CVD) method and a facile low-cost surface oxidation process. The morphologies, crystal structures, and element composition of the as-fabricated core/shell NWs were characterized. Photoluminescence (PL) properties of them were studied. A possible growth mechanism was proposed. The results show that the as-fabricated Zn3P2/ZnO core/shell NWs Received: September 3, 2010 Revised: March 11, 2011 Published: March 30, 2011 1417

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Crystal Growth & Design have substantial potential for future photovoltaic nanodevice applications. The fabrication process of the Zn3P2/ZnO core/shell NWs involved two steps as shown in Scheme 1. A detailed experimental section is available in the Supporting Information. Figure 1a depicts a field emission scanning electron microscopy (FESEM) image of the as-synthesized Zn3P2 NWs, which reveals that the NWs are hundreds of micrometers in length. The upper inset is an optical image of the product, which indicates that the as-synthesized Zn3P2 NWs are of high yield. There are two Scheme 1. Schematic Illustration of the Two-Step Fabrication Process of the Zn3P2/ZnO Core/Shell NWs

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typical morphologies of the Zn3P2 NWs in the product: straight NWs and zigzag NWs. Typical transmission electron microscopy (TEM) (left panel) and high-resolution transmission electron microscopy (HRTEM) (right panel) images of a straight Zn3P2 NW are shown in Figure 1b. From the TEM image, it is clear that the Zn3P2 NW has a smooth surface and uniform diameter along its entire length. From the HRTEM image, crystal planes with spacing distances of about 0.33 and 0.40 nm can be seen clearly along and perpendicular to the growth direction, respectively. According to the JCPDS (JCPDS card No. 65-2854) data, the corresponding planes can be indexed as the tetragonal Zn3P2 (020) and (202) planes, respectively. Figure 1c is a typical TEM image and HRTEM images of a zigzag Zn3P2 NW. From the HRTEM image, where the Zn3P2 (202) planes with a spacing distance of 0.33 nm are labeled, we can see that the zigzag Zn3P2 is twinned crystal.24 The inset of Figure 1d shows a typical energy dispersive X-ray analysis (EDX) spectrum taken from a straight or zigzag Zn3P2 NW. It consists mainly of Zn and P signals with an atomic ratio of about 3:2. The signals of Cu and C are from the copper grid used for TEM observation. The small amount of O should be from the surface amorphous layer, as we can see that the surface of either straight or zigzag Zn3P2 NW is covered by a thin amorphous layer. The EDX spectra taken from this amorphous layer consist mainly of Zn, P, and O signals (not shown here). The surface reactivity of Zn3P2 is a well-known phenomenon, especially in O2 atmosphere and under high temperature.25 A typical PL spectrum (Figure 1d) of an individual straight or

Figure 1. (a) Low-magnification FESEM image of the Zn3P2 NWs. Inset: an optical image of the product. TEM (left panel) and HRTEM (right panel) images of a straight (b) and zigzag (c) Zn3P2 NW. (d) Typical room-temperature PL spectrum and EDX spectrum (inset) of an individual straight or zigzag Zn3P2 NW. 1418

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Figure 2. TEM (left panel) and HRTEM (right panel) images of a straight (a) and zigzag (b) Zn3P2/ZnO core/shell NW obtained after surface oxidation of Zn3P2 NWs under 400 °C for 1 h. (c) Line-scanning elemental mappings of P and O on a straight core/shell NW along the route indicated by the red line in the inset. Inset: the HAADF-STEM image of the core/shell NW. (d) HRTEM image of Zn3P2/ZnO core/shell NW after oxidation under 400 °C for 3 min.

zigzag Zn3P2 NW shows a strong PL emission centered around 770 nm (1.61 eV), which corresponds to the near-bandedge emission of Zn3P2.26 After oxidation, the obtained Zn3P2/ZnO core/shell NWs keep the morphologies of their mother Zn3P2 NWs. Figure 2a (left panel) exhibits a typical TEM image of a straight Zn3P2/ ZnO core/shell NW. The clear contrast variation along the radial direction of the NW confirms the formation of core/shell structure. The diameter of the core is about 150 nm, and the thickness of the shell is about 30 nm. An HRTEM image taken at the shell area is shown on the right panel of Figure 2a. We can see clearly the formation of nanocrystals at the shell. The 0.28 and 0.52 nm lattice spacings labeled in this figure correspond to the spacing distance of hexagonal ZnO (1010) and (0001) planes, respectively. Figure 2b displays TEM image and HRTEM image

of a zigzag Zn3P2/ZnO core/shell NW. Again, the contrast variation between the core and the shell, and the formation of ZnO nanocrystals in the shell can be seen clearly. Besides the hexagonal ZnO (1010) planes, the hexagonal ZnO (0002) crystal planes with a spacing distance of about 0.26 nm are indexed in the HRTEM image. In order to further investigate the spatial distribution of the element composition along the radial direction of the core/shell NW, line-scanning elemental mappings of P and O were conducted on a straight core/shell NW (Figure 2c). The inset is the high-angle angular darkfield scanning transmission electron microscopy (HAADF-STEM) image of the NW. The line scanning was performed along the red line. We can see that O and P elements locate mainly in the shell and core regions, respectively. In order to reveal the growth mechanism of the Zn3P2/ZnO core/shell NWs, we investigated the crystal structure of the shell at its earlier 1419

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stage, by carrying out a quick surface oxidization process (400 °C for 3 min). The corresponding HRTEM image of the Zn3P2/ZnO core/shell NW is shown in Figure 2d. We can see clearly the formation of the ZnO nanocrystals in the amorphous layer on the surface of Zn3P2 NW. The 0.26 nm lattice spacings labeled in the shell correspond to the spacing distance of hexagonal ZnO (0002) planes. Because the shell is thin, here we can get a clear HRTEM image of the Zn3P2 core, which remains to be single-crystalline. The tetragonal Zn3P2 (020) planes (with the spacing distance of 0.33 nm) and the (404) planes (with the spacing distance of 0.20 nm) are labeled. We suggest a possible growth mechanism of the ZnO nanocrystal shell on the Zn3P2 NW as Reactions I
III given below: Zn3 P2 þ O2 f ZnO ðamorphousÞ þ Py Ox ðsolidÞ Py Ox ðsolidÞ f Py Ox ðgasÞ

ð400°CÞ

ZnO ðamorphousÞ f ZnO ðnanocrystallineÞ

ðIÞ ðIIÞ

ð400°CÞ ðIIIÞ

The surface of Zn3P2 NW will be oxidized in O2 atmosphere, yielding amorphous ZnO and PyOx solid compounds (eq I). Under 400 °C, the PyOx solid compounds will sublimate, and ZnO nanocrystals will form (corresponding to an annealing process) in the amorphous layer (Reactions II and III). Since the melting point of Zn3P2 core is about 420 °C, the oxidization process can not be carried out at the temperature higher than 420 °C. Figure S1 in Supporting Information shows a typical HRTEM image taken at the ZnO shell synthesized at 300 °C. Compared with its counterpart shown in Figure 2a,b, it is clear that the quality of ZnO nanocrystal shells synthesized at higher temperatures are better. This may be because a higher temperature would lead to better crystallization of ZnO. Typical room-temperature PL spectrum for an individual straight or zigzag Zn3P2/ZnO core/shell NW is shown in Figure 3a. There are three distinct peaks in this spectrum. The 383 nm (3.24 eV) and 770 nm (1.61 eV) sharp peaks are well explained as the near-bandedge emissions of ZnO and Zn3P2, respectively. The peak at 517 nm may come from two aspects: (1) the oxygen-vacancies or surface-states related emission from ZnO, which is commonly reported in many works;27
29 (2) the existence of ZnxPyOz owing to the miscibility of Zn3P2/ZnO. However, the existence of the clear band edge emission peaks of pure ZnO (383 nm) and Zn3P2 (770 nm) indicates that the miscibility of these two substances is small. Experimental work on the miscibility of Zn3P2/ZnS showed that the miscibility is quite small on the order of 1%.30 Corresponding theoretical work drew the same conclusion.31,32 On the basis of the above consideration, we think the peak at 517 nm may mainly result from the oxygen-vacancies or surfacestates related emission from ZnO. It is worth noting that compared with that of Zn3P2 NW without the ZnO shell (Figure 1d), the PL intensity of the Zn3P2 NW core covered with ZnO nanocrystalline shell is markedly weaker. In order to explain this phenomenon, we plot the schematic VBM and CBM alignments of a Zn3P2/ZnO core/shell NW, which belongs to a type-II heterostructure. From this diagram, we can see that under light excitation, the photogenerated electrons and holes will be confined mainly in the ZnO shell and in the Zn3P2 core, respectivley. Therefore, the recombination rate of the electrons and holes will decrease, resulting in a weaker PL intensity.13,14,29 Nevertheless, the minority carrier lifetime in this heterostructure will increase.13 Furthermore, the

Figure 3. (a) Typical room-temperature PL spectrum of an individual straight or zigzag Zn3P2/ZnO core/shell NW. (b) Schematic conduction band minimum (CBM) and valence band maximum (VBM) alignments of a Zn3P2/ZnO core/shell NW, which belongs to typeII heterostructure.

rough surface of the ZnO nanocrystalline shell can create multiple reflections and enhance the light absorption efficiency.33 The above merits together with the ideal bandgaps of both Zn3P2 and ZnO will benefit the Zn3P2/ZnO core/shell NWs to be used in the photovoltaic nanodevices in the future. In summary, novel Zn3P2/ZnO core/shell NWs were synthesized for the first time by combining a catalyst-free CVD method and a facile low-cost surface oxidation process. The morphologies, crystal structures, and element composition of the obtained core/shell NWs were characterized by FESEM, HRTEM, and HAADF-STEM. Room-temperature PL properties of them were studied. A possible growth mechanism was proposed. The heterostructure fabrication approach developed here may be extended to other functional heterostructure material systems. The experimental results together with energy band analysis demonstrated that the Zn3P2/ZnO core/ shell NW type-II heterostructures have substantial potential for future photovoltaic nanodevice applications.

’ ASSOCIATED CONTENT

bS

Supporting Information. Detailed descriptions of experimental procedures and characterization, a typical HRTEM image taken at the ZnO shell synthesized at 300 °C, and a method to use this core/shell material for solar cell related work are available free of charge via the Internet at http://pubs.acs.org. 1420

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 11074006, 10774007, 10874011, 50732001), the National Basic Research Program of China (Nos. 2006CB921607, 2007CB613402), and the Fundamental Research Funds for the Central Universities.

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(29) Wang, K.; Chen, J. J.; Zeng, Z. M.; Tarr, J.; Zhou, W. L.; Zhang, Y.; Yan, Y. F.; Jiang, C. S.; Pern, J.; Mascarenhans, A. Appl. Phys. Lett. 2010, 96 (123105), 1–3. (30) Locmelis, S.; Binnewies, M. Z. Anorg. Allg. Chem. 2004, 630, 1301. (31) Jug, K.; Gloriozov, I. P.; Heidberg, B. J. Phys. Chem. B 2005, 109, 21922–21927. (32) Jug, K.; Nair, N. N.; Gloriozov, I. P. J. Phys. Chem. B 2006, 110, 4111–4114. (33) Chao, H. Y.; Cheng, J. H.; Lu, J. Y.; Chang, Y. H.; Cheng, C. L.; Chen, Y. F. Superlattices Microstruct. 2010, 47, 160–164.

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