Indium-doped ZnO nanowires with infrequent growth orientation, rough surfaces and low-density surface traps
© Duan et al.; licensee Springer. 2013
Received: 29 August 2013
Accepted: 17 November 2013
Published: 21 November 2013
Indium-doped ZnO nanowires have been prepared by vapor transport deposition. With increasing In content, the growth orientation of the nanowires switches from  to infrequent  and the surface becomes rough. No surface-related exciton emission is observed in these nanowires. The results indicate that large surface-to-volume ratio, high free electron concentration, and low density of surface traps can be achieved simultaneously in ZnO nanowires via In doping. These unique properties make In-doped ZnO nanowire a potential material for photocatalysis application, which is demonstrated by the enhanced photocatalytic degradation of Rhodamine B.
One-dimensional (1D) ZnO nanostructures have attracted extensive research interests in the past decade due to their versatile application potential in nanooptoelectronics , electromechanics , and catalysis . It has been found that doping impurities, especially group III elements, such as Al , Ga , In , can significantly enhance the electrical conductivity and influence the optical properties. In order to generate desirable electrical, optical, and catalytic properties, 1D ZnO nanostructures have been doped with selected elements. Among these dopants, In is recognized as one of the most efficient elements used to tailor the optoelectronic properties of ZnO . For example, In doping may induce structural defects such as stacking faults , twin boundaries , and superlattice structures , or result in weak localization and electron–electron interactions , which can significantly affect the electrical and photoluminescence (PL) properties of ZnO nanostructures. On the other hand, it is quite interesting that In doping can change the morphology of ZnO nanowires (NWs) . There are three typical fast-growth directions (, , and ) and ± (0001) polar surfaces in wurtzite ZnO . In general, ZnO NWs grow along  direction. When doped with In, however, they may grow along some other directions, such as the non-polar  direction .
ZnO nanostructures usually have plenty of surface states acting as carrier traps. The existence of such traps is unwanted in catalytic applications, which take advantage of free carriers in the surface region of ZnO nanostructures. In this regard, ZnO nanostructures with large surface-to-volume ratio, high free electron concentration, and low density of surface traps are highly desired.
In this work, we demonstrated that such ZnO nanostructures can be achieved via In doping. The In-doped ZnO NWs were grown by one-step vapor transport deposition. The effect of In doping content on the morphology, structure, and optical properties of the NWs has been investigated. With increasing In doping content, ZnO NWs show switches of the orientation from  to an infrequent  direction and surface from smooth to ripple-like. Low-temperature PL spectra indicate that indium indeed acts as shallow donor and the density of surface traps is very low. We demonstrated the enhanced photocatalytic performance of In-doped ZnO NWs by degradation of Rhodamine B (RhB) solution.
The In-doped ZnO nanowires were synthesized by a vapor transport deposition process in a single-zone high-temperature tube furnace. A mixture of ZnO (99.999%), graphite (99.9%), and In2O3 (99.99%) powder (weigh ratio 8:2:1) was used as the source material. A layer of 5-nm gold film deposited on the Si (100) substrate before the growth of ZnO NWs was used as catalyst. Then the treated silicon substrate and the source material were placed in a quartz boat and inserted into the tube furnace. Si (100) substrate was placed about 10 cm downstream of the source. Before growth, the quartz tube was evacuated to about 100 mTorr by a rotary pump. Then the tube furnace was heated to 950°C at a rate of 20°C min−1, under a Ar flow rate of 100 standard-state cubic centimeter per minute (SCCM). When the temperature reached 950°C, high purity O2 was continuously fed into the tube at a flow rate of 2 SCCM, and the pressure was maintained at 4 Torr. After reacting for 30 min at 950°C, the furnace was naturally cooled to room temperature without O2 flux, and the white product deposited on the silicon substrate was collected. Undoped ZnO NWs were also grown under the same experimental conditions.
The structure and composition of the samples were analyzed by X-ray diffraction (XRD) through a Rigaku D/max 2550 pc diffractometer (The Woodlands, Texas, USA) and secondary ion mass spectroscopy (SIMS) on a time-of flight mass spectrometer (Ion TOF-SIMS). The morphology and microstructure of the nanowires were characterized by scanning electron microscopy (SEM, Hitachi S-4800, Tokyo, Japan) and transmission electron microscopy (TEM, Philips-FEI Tecnai G2 F30 S-Twin, Hillsboro, OR, USA) combined with selective area electron diffraction (SAED). The In doping content of the individual NW was confirmed by energy dispersive X-ray spectroscopy (EDX) equipped in the TEM instrument. PL spectra were measured on a fluorescence spectrometer (FLS920 Edinburgh Instruments, Livingston, West Lothian, UK), using a He-Cd 325-nm laser as the excitation source.
The photocatalytic activity of the nanowires was evaluated by investigating the photocatalytic degradation of RhB in aqueous solution in a cylindrical quartz photoreactor. Thirty milligrams of each sample was dispersed in 100 ml of deionized water, followed by ultrasonication for 1 h. One milliliter of 1 mM RhB aqueous solution was then added. A Xe lamp was used as the illumination source. Before illumination, the solution was stirred continuously in the dark for 30 min to reach an adsorption-desorption equilibrium of dye molecules on the surface of photocatalysts. The concentration of the remaining dyes was monitored by measuring the absorbance of the solution using a UV–vis spectrophotometer (Shimadzu 3600, Tokyo, Japan).
Results and discussion
From the TEM images (Figure 3c,d), we can observe that the high-content In-doped ZnO NWs have ripple-like surface, which can result in a much larger surface-to-volume ratio and thus facilitate the formation of SXs. Therefore, remarkable surface state-related emission would have been expected in our sample. However, no SX-related emission peak (approximately 3.366 eV) is observed in the low-temperature PL spectrum of sample #3, as shown in Figure 4a. Moreover, the deep level emission, which is found to largely originate from surface defects , decreases with increasing In-doping concentration (Figure 4b). These results indicate that the influence of the surface states on the PL properties of sample #3 is almost negligible, which strongly suggests that the density of surface electron traps is at a very low level in our sample.
In summary, the morphology, microstructure, and PL properties of In-doped ZnO NWs prepared by vapor transport deposition method were investigated. The nanowires exhibit switches of the orientation from  to an infrequent  direction and the surface from smooth to ripple-like with increasing In doping content. The ZnO NWs with In content of 1.4 at.% have large surface-to-volume ratio with lateral surfaces formed by (100) and (101) facets. Low-temperature PL shows two dominant emissions at 3.357 and 3.31 eV, indicative of the formation of InZn donors and stacking faults, respectively. The In-doped ZnO NWs do not show surface exciton emission, which indicates a low density of surface electron traps in our samples. We demonstrate that ZnO NWs with large surface-to-volume ratio, high electron concentration, and low-surface trap density can be achieved simply by In doping, which are desirable for efficient photocatalysis.
This work was financially supported by the Natural Science Foundation of China under Grant nos. 51172204 and 51372223, Science and Technology Department of Zhejiang Province Project no. 2010R50020.
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