Nanoscale Chemical Imaging of Zinc Oxide Nanowire Corrosion - The

Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee ... Publication Date (Web): April 16, 2012 ... For a more comprehensiv...
0 downloads 0 Views 4MB Size
Download PDF
Article pubs.acs.org/JPCC

Nanoscale Chemical Imaging of Zinc Oxide Nanowire Corrosion K. A. Cimatu, S. M. Mahurin, K. A. Meyer, and R.W. Shaw* Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ABSTRACT: The ability to monitor corrosion on a nanometer scale is a powerful tool for a fundamental understanding of surface chemical processes. Nanoscale chemical images of individual bare and alumina-coated zinc oxide nanowires (NWs) were recorded using tip-enhanced second harmonic generation (SHG) spectroscopy before and after exposure to carbon dioxide and water vapor. Images were collected for the same bare nanowire after each successive two-day exposure period. Corrosion of the bare ZnO NW to zinc carbonate was evident from far-field and near-field SHG images and simultaneously recorded atomic force microscopy (AFM) data. The expected zinc carbonate corrosion product is SHG inactive. The AFM profile of the NW showed vertical and lateral expansion in different regions of the nanowire. The lower resolution far-field SHG signal decreased gradually and uniformly. The near-field SHG signal provided a profile of the evolving NW with a spatial resolution approaching 100 nm. In contrast, exposed alumina-coated ZnO NWs showed reduced, but still observable, degradation. The 3 nm thick alumina protective layer may have been insufficient to fully protect the NW, or the coating may have been incomplete. Thicker coatings preclude the tip-enhanced method. Further nanometer-scale imaging should lead to the discovery of protective layers to prevent or delay ZnO degradation.

■

INTRODUCTION Semiconducting zinc oxide is synthesized as bulk material, thin films,1 or nanostructures depending on the application for solar energy cells,2−4 light-emitting diodes,5 lasers,6,7 field-effect transistors,8 sensors,9 photodetectors,10 and other optoelectronic applications.11 The ZnO wide bandgap energy and large exciton binding energy enhance light emission efficiency.5,9,12 An important aspect of zinc oxide as a semiconductor is that it is an environmentally friendly material whose components are abundant in the earth’s crust. Since ZnO is inexpensive, easy to prepare, biocompatible, and biodegradable, it is also used in drugs and sunscreen protection products. However ZnO, particularly nanotextured ZnO, is not stable in ambient atmospheres that contain carbon dioxide and water vapor.13 This instability toward formation of zinc carbonate has the potential to spoil important applications that depend on the ZnO semiconducting properties. Thus, we have chosen to examine this reaction using a technique that allows us to follow surface chemistry and morphology changes at the nanoscale for ZnO nanowires. In particular, we wished to follow the changes of a single nanowire as it transformed. Various growth methods have been used to produce zinc oxide nanostructures, including growth based on the vapor− liquid−solid mechanism, thermal evaporation, gas-phase reactions, and chemical vapor deposition.14,15 For zinc oxide nanowires, extensive effort has been devoted to control their orientation, position, diameter, and morphology.16 At the same time, several techniques have been applied to characterize the material properties, including optical properties, using scanning electron microscopy, transmission electron microscopy, and © 2012 American Chemical Society

near-field (NF) scanning optical microscopy (NSOM). NSOM was used to study the lasing properties of single nanowires.17,18 ZnO (Zincite) is not stable in the atmosphere. It slowly reacts with water and carbon dioxide to form zinc carbonate. Initially, ZnO reacts with water to form zinc hydroxide, which then reacts with CO2 forming zinc carbonate, ZnCO3.13 Zinc oxide can be converted either into Zn4CO3(OH)6·H2O in the presence of >1 ppm CO 2 or into hydrozincite (Zn5(CO3)2(OH)6) with 40 000 ppm CO2. The formation of these corrosion byproducts is greatly dependent on the available concentration of CO2.19 On the basis of the literature, if Zn metal is exposed in air but in a CO2-free environment, ZnO is formed. On the other hand, Zn metal exposure to CO2 leads to formation of a surface layer of coarse crystals corresponding to Zn4CO3(OH)6·H2O as identified by X-ray diffraction. A significant gain in mass after exposure to carbon dioxide, especially at higher concentrations, was also found.19 It was also observed that as the CO2 concentration was increased the rate of corrosion increased. The corrosive effect of CO2 was suggested to be due to acidification of the surface water. Thus, an enhanced dissolution of the ZnO surface to form zinc carbonate was observed. This reaction or degradation is a form of corrosion. Formation of the carbonate degrades the electronic and optical performance characteristics of ZnO.19,20 This corrosion reaction is more severe for ZnO nanostructures, owing to their large surface areas. On the basis of the findings of Paranthaman and his co-workers, aged ZnO nanowire Received: February 27, 2012 Revised: April 12, 2012 Published: April 16, 2012 10405

dx.doi.org/10.1021/jp301922a | J. Phys. Chem. C 2012, 116, 10405−10414

The Journal of Physical Chemistry C

Article

surfaces are coated with a thin layer of ZnCO3,13 which leads to morphological changes and modifications of both the chemical and physical properties. The morphological changes may appear as lateral (parallel to the substrate surface), vertical (normal to the substrate surface), and pendant nanorod ZnCO3 growth from a ZnO NW core or even etching away of the ZnO NW. The formation of a surface layer will initially prevent the inner ZnO NW core from reacting, but since the ZnCO3 films are less compact (the density of ZnO is 5.61 g/cm3 while ZnCO3 is 4.35 g/cm3), corrosion of the zinc oxide inner core will continue.13 Attempts have been made to prevent ZnO degradation13 by creating a protective coating over ZnO films or nanostructures, specifically using films that are not reactive with carbon dioxide and water vapor. Titania and alumina have been used to coat ZnO NWs via atomic layer deposition (ALD).13,21 The stability of the coated ZnO NWs was studied21 while they underwent photocorrosion under ambient conditions, and a negligible difference was noted between the bare and alumina-coated NWs after a two-year period.21 Second harmonic generation (SHG) is a second-order nonlinear process that has been used to study systems with nanoscale dimensions.22,23 In SHG, the frequency of incoming laser light is doubled, as it interacts with a nonlinear material. This summation process generates new photons with twice the energy, i.e., twice the frequency, and half the wavelength of the incident photons.24 The second harmonic (SH) intensity (ISH) is proportional to the square of both the material second-order nonlinear susceptibility (χ(2)) and the excitation light intensity. For amorphous materials or those with a symmetry where χ(2) = 0, ISH is zero, unless a surface generation process (where χ(2) is always finite) contributes a small signal. SHG is the degenerate case of the sum frequency generation nonlinear optical process.25 Since SHG is a coherent process, the output photon beam is directional, permitting efficient collection of the signal photons. SHG has been combined with NF scanning optical microscopy (NSOM) methods, specifically apertureless tip-enhanced SHG microscopy.26 The metal tip apertureless NSOM method is an ideal tool for studying the morphology of nanoscale materials with subdiffraction limited resolution. The intensity of the SHG signal is dependent on the metal-coated atomic force microscopy (AFM) probe apex which is described as a point source for enhancement, where the laser wavelength is resonant with optical plasmons in the metal.27 Usually, in any far-field (FF) approach, SHG images are diffraction-limited; thus, for those methods, nanoscale areas that can contain significant details that might contribute to an understanding of chemical processes at a nanoscale level may be overlooked. Greater spatial resolution can be achieved by using a NF, tip-enhanced method where spatial resolution can be improved to the order of the tip radius (20−100 nm typically) and where a signal enhancement of about 2000-fold can be attained.26 In this paper, we report the surface degradation of ZnO NWs with nanoscale spatial resolution. A ZnO NW was characterized before and after exposure to carbon dioxide and water vapor using a NF, tip-enhanced SHG imaging microscope. This nonlinear imaging instrument26 provides FF, NF, and AFM images of nanoscale objects. The resolution of the microscope allowed discovery of an unexpected nonuniform degradation of the ZnO NW. The data that will be shown here illustrate that in different regions along the NW length the NW expanded vertically or laterally upon conversion of the ZnO to ZnCO3.

Alumina-coated ZnO NWs with different coating thicknesses were also prepared and studied using the tip-enhanced SHG method. We observed that for thicker coatings tip enhancement of the SHG of the underlying ZnO core diminished due to the limited approach of the tip to the core. We were able to study corrosion for ZnO NWs with a 3 nm thick protective coating.

■

EXPERIMENTAL SECTION ZnO Nanowire Preparation by Thermal Evaporation. ZnO nanowires were prepared from a mixture of 2 g of ZnO powder and 0.02 g of Zn metal.28 The two solids were mixed in a quartz boat, which was inserted into a tube furnace and positioned at the hot zone center of the tube. Three rectangular pieces of silicon wafer were placed at the exit of the heated zone to collect the ZnO NWs from thermal evaporation/ condensation. The tube furnace was evacuated to 10 mTorr, and Ar gas was admitted at a 35 sccm flow rate. The furnace was turned on and the temperature ramped to 1200 °C over 1 h. The furnace was turned off at 2 h. Once the temperature dropped to 200 °C, the furnace cover was removed for faster cooling. When room temperature was reached, the Ar gas and the vacuum pump were turned off. Nanowire Characterization Using SEM and TEM. ZnO NW samples were characterized using scanning (SEM) and transmission (TEM) electron microscopy.13 A scanning electron microscope (JEOL 6060) was used to determine the size and shape of the ZnO nanowires deposited on clean Si wafers. Depending on the collection wafer position (i.e., temperature) in the tube furnace, different lengths, sizes, and shapes of nanowires were obtained, with a diameter ranging from 30 to 300 nm. A transmission electron microscope (Hitachi HD2000 scanning TEM) operating at 200 kV was used to record images of the alumina-coated ZnO nanowires. Alumina Coating of ZnO Nanowires by Atomic Layer Deposition (ALD). The NWs on the Si wafer were transferred to an ethanol suspension by sonication. Four drops (0.2 mL) of the suspension were placed on a 0.16 mm thick quartz coverslip (Quartz Plus, Inc., Brookline, NH; 25 mm × 25 mm × 0.16 mm), which was then dried in air. The deposition of an alumina layer was carried out in a viscous-flow ALD chamber reactor at a temperature of approximately 65 °C.29 Trimethylaluminum (TMA) (Sigma-Aldrich, St. Louis, MO) and high-purity water were alternately introduced into the reaction chamber using dry nitrogen as a carrier gas. The nitrogen flow was maintained at a 30 sccm flow rate which yielded a chamber pressure of 1 Torr. One ALD cycle was 42 s in duration and consisted of the following exposure times: (i) 1 s TMA, (ii) 20 s N2 purge, (iii) 1 s H2O, and (iv) 20 s N2 purge. A 3 nm coating was obtained after 18 cycles. Finally, the chamber was pumped to remove excess precursor materials, and the sample was removed.21,29,30 This technique was used to prepare 3 and 6 nm thick coatings. Sample Preparation on Quartz Coverslips. The nanowires were removed from the Si growth substrate by sonicating for 5 min in ethanol. Four drops of the resulting suspension were placed on a clean quartz coverslip (0.16 mm thick) that was subsequently air-dried. The coated coverslip was attached to an aluminum sample ring in preparation for atomic force microscopy scanning and FF and NF imaging measurements. SHG Microscope. The instrument for AFM/FF- and NFSHG microscopy26 utilized a 250 kHz Ti:Sapphire regenerative amplifier (800 nm, 150 fs pulsewidth) as the excitation source, with a typical pulse energy of 2.6 μJ. The laser beam was focused through a 10× objective lens coupled to a 100 μm core 10406

dx.doi.org/10.1021/jp301922a | J. Phys. Chem. C 2012, 116, 10405−10414

The Journal of Physical Chemistry C

Article

Figure 1. (A,B) Scanning electron microscope (SEM) images of freshly prepared ZnO nanowires on a silicon wafer and (C) transmission electron microscope image of an alumina-coated ZnO nanowire after ALD. The scale bars are 5 μm, 200 nm, and 50 nm, respectively.

multimode optical fiber. The other fiber end was coupled to an inverted fluorescence microscope (TE300, Nikon) using a 10× objective that directed the recollimated beam along the optical axis of the microscope. Before reaching the sample surface, the laser beam propagated through the following optics: (a) polarization scrambler; (b) rotatable linear polarizer; (c) 800 nm bandpass filter (10 nm fwhm); (d) dichroic filter (XF2019, Omega Optical); and (e) 100× NA = 1.49 oil immersion objective (Nikon Instruments Inc., Melville, NY). The laser power was adjusted to obtain an adequate signalto-noise ratio for the SHG signal but to avoid sample degradation over the long duration of the corrosion experiment. The laser beam pulse energy was typically 44 nJ at the sample in this epi-illumination geometry. The laser power was checked before every measurement to maintain consistency. Epiillumination was employed to minimize formation of a diffraction pattern at the sample that would have resulted in variable illumination intensity across the sample. Three types of data sets were recorded using the microscope. First, the 400 nm FF-SHG image from a selected ZnO nanowire was collected through the microscope objective, a dichroic filter (XF2019, Omega Optical), and a 10 nm fwhm 400 nm bandpass filter and detected using an ICCD camera (PI-MAX 1K, 1024 × 1024 pixel array, Princeton Instruments). This image was utilized to choose a particular ZnO nanowire to be characterized. Second, the NF-SHG sample image was collected using the same detector, in combination with an AFM/apertureless NSOM (model MV4000, Nanonics Imaging, Jerusalem, Israel) mounted on the inverted microscope stage. The topographic image of the nanowire from the same tip raster comprised the third data set. The cantilevered silica AFM tip used was gold-coated (20 nm thick), had a 60 nm overall apex diameter, and made a 90° tip to surface angle. The gold-coated tip was utilized to realize tip-enhanced NF-SHG images of a selected NW. The tip was engaged at the sample surface, and apertureless SHG images were recorded by fixing the sample in place and scanning the tip over the nanowire locale. The selected NW long axis was orthogonal to the tip scan axes. For each AFM scan pixel, the ICCD camera pixel intensity values were summed for a specified region-of-interest (ROI) that corresponded to the diffraction-limited NW image (64 × 64 camera pixels). Then the summed ROI intensities plotted against the x and y AFM raster coordinates is a map of SHG enhancement factors relative to the nominal NW SHG intensity, i.e., a NF-SHG image of the NW. The spatial resolution of this method approached 100 nm, not substantially larger than the tip diameter. The tip step parameter 0.031 μm was set to create 2 × 2 μm images; the 64 × 64 pixel image required 2 h of acquisition time.

To acquire a dispersed SHG spectrum of a single nanowire, the 400 nm (10 nm fwhm bandpass) filter was replaced with a Schott BG-39 filter with a broader transmission curve but that still blocked 800 nm excitation light; then, at a separate microscope port, a 200 μm multimode fiber collected the SHG photons. The distal end of the fiber was connected to a spectrograph/CCD camera input port. Additional details of the experimental setup were described previously.26 Carbon Dioxide and Water Vapor Exposure. A chamber was configured to surround the microscope sample stage. The CO2 exposure gas flowed into the plexiglass chamber at 200 sccm. An open beaker of water in the chamber provided the H2O partial pressure. The carbon dioxide gas had a ∼26:1 volume ratio with the water vapor, as determined from the water volume loss from the beaker over the exposure period. This concentration was much higher than that for Yang’s exposure.21 The exposure was performed in two-day increments. Approach. Two coverslips were positioned on the SHG microscope26 stage, one loaded with bare ZnO NWs and the other with alumina-coated NWs. We analyzed an unexposed single alumina-coated ZnO nanowire using atomic force microscopy and near-field second harmonic generation microscopy, and after two hours of image acquisition, the sample was removed. Then for comparison, a single uncoated ZnO NW was aligned in the microscope, and the same analysis was performed. After these preliminary experiments were performed, the corrosion experiment was performed for the same single bare ZnO nanowire by exposing the NW to carbon dioxide and water vapor at a high chamber concentration without moving the sample slip. A high CO2/H2O concentration was used to induce faster corrosion to complete the experiment within two weeks. The alumina-coated ZnO NW sample was also present in the same chamber to ensure comparable exposure. The samples were exposed for two days, and then the plexiglass chamber was removed to perform the imaging analysis, avoiding any sample disturbance. The same single ZnO nanowire was reimaged. After analysis, the chamber was replaced for another two-day exposure. This procedure was repeated to obtain a total of six days of exposure. On the last day of exposure, the alumina-coated ZnO NW sample was returned to the microscope focus and analyzed, but a different coated NW was necessarily examined due to the sample relocation.

■

RESULTS AND DISCUSSION SEM and TEM images of bare (uncoated) and alumina-coated ZnO NWs are shown in Figure 1. The SEM image of the NWs provided size information and also displayed the form of the 10407

dx.doi.org/10.1021/jp301922a | J. Phys. Chem. C 2012, 116, 10405−10414

The Journal of Physical Chemistry C

Article

material. The TEM image of the alumina-coated NW showed an approximately 3 nm alumina coating. Figure 2A is a topographic AFM image of a single ZnO nanowire where the scanning probe tip was rastered in a 2 μm × 2 μm pattern. The maximum NW height (diameter) was approximately 100 nm. Figure 2A is actually an average of matching trace and retrace tip scans over the NW, yielding

slightly better S/N than each individual scan. Figure 2B illustrates the nonlinear optical NF image of the unexposed NW that was collected simultaneously with the Figure 2A AFM image. A custom Labview (National Instruments, Austin, TX) program was used to collect the SHG intensity for a specified ICCD ROI (see Figure 2C) for each AFM scan pixel, and then the data were remapped as trace and retrace images. Figure 2B is an average of the trace and retrace scans. The SHG signal strength was greater when the gold-coated AFM tip was directly over the nanowire. When the tip was not on the NW, the SHG signal was lower. Thus, the resultant “enhancement” image displays a single NW at subdiffraction limited spatial resolution. Figure 2C is the NW FF image. The nanowire has a high aspect ratio form with approximately 2 μm length. The apparent NW rotation between Figures 2A and 2C is due to the AFM scan orientation at 45° to the microscope axes. The NF-SHG intensity variation along the wire could be due to the following reasons: (a) laser intensity or wavelength drift during the two hours of data acquisition for a single image, (b) nonuniform 800 nm beam profile, and (c) the zinc oxide may have already begun to degrade at one end of the NW. A cartoon illustration is shown in Figure 3 to help visualize ZnO corrosion resulting in the formation of amorphous ZnCO3. Figure 3 illustrates different structural modes of zinc carbonate formation based on a literature report,13 including formation of pendent zinc carbonate nanorods. Figure 3B,C shows that there can be symmetric or asymmetric formation of a ZnCO3 thin film; the latter may explain the nonuniform enhancement present in Figure 2B, indicating early ZnO NW degradation.13 The FF-SHG image of Figure 2C indicates that the overall ZnO NW is uniform; however, the FF-SHG image probably lacks sufficient sensitivity to report few-nanometer changes corresponding to early corrosion. The zinc oxide NW shown in Figure 2 was then exposed to carbon dioxide and water vapor inside a plexiglass chamber that enclosed the sample. The exposure was carried out in two-day intervals for a total of six days. After each exposure interval, the same single ZnO NW was characterized using the same goldcoated AFM tip; the sample slide was kept on the microscope stage during the entire experiment. Figure 4A−C shows the AFM/NF/FF data after the NW of Figure 2 was exposed for two days. The image data for Figures 4 and 5 is also summarized in Table 1 for ease of comparison. The entries in the FF-SHG column of Table 1 were derived by averaging the image pixel values for a ROI that just enclosed the NW. Figure 4A is a topographic image of the nanowire, where nonuniformity along the NW length is clearly evident. Box 1 in Figure 4A shows a region where the NW height decreased from the initial ∼100 to ∼50 nm. This decrease in height is due to chemical degradation of the zinc oxide NW. As reported by Pan et al.,13 production of zinc carbonate resulted in a change of the NW shape due to formation of a ZnCO3 layer around the reduced diameter ZnO NW core. Growth of ZnCO3 was lateral (broadening) or vertical (increased height). The preferred lateral growth might be due to an effect of the substrate material on the stability of the ZnO NW. As reported by Pan et al., untreated substrates such as glass slides enhance the atmospheric corrosion of ZnO NWs. Water vapor being abundant in air can be adsorbed dissociatively on the oxide surface of the substrates, thus forming active sites (MOH,  M−O(H)−M, etc., where M is the substrate Si atom in our experiment).31 Since our experiment was performed under ambient conditions, the oxide layer should always have been

Figure 2. (A) AFM image (color bar indicates z-axis/height in nm), (B) tip-enhanced NF-SHG image (color bar indicates intensity in au), and C) FF image (color bar indicates SHG intensity in au) of a bare ZnO nanowire before exposure to carbon dioxide and water vapor (0 day exposure). Note that in (C) a region-of-interest (ROI) box outline (dashed line, ---) is presented to represent the data summation area of the NF-SHG experiments. The figure color scales have been adjusted for minimum amplitude of zero. 10408

dx.doi.org/10.1021/jp301922a | J. Phys. Chem. C 2012, 116, 10405−10414

The Journal of Physical Chemistry C

Article

Figure 3. Cartoon representation of ZnO nanowire degradation after exposure to carbon dioxide and water vapor, resulting in different morphologies of the ZnCO3 product. The new NW features are based on TEM images from Pan’s paper.13 (A) Uncorroded ZnO NWs on a quartz coverslip, (B) symmetric formation of a ZnCO3 thin film on a ZnO NW core, (C) asymmetric growth of a ZnCO3 film along the ZnO NW length, (D) ZnCO3 nanorod clusters growing from the ZnO NW surface, (E) ribbon-like/flattened ZnCO3 nanostructures, and (F) uneven etching of the ZnO.

hydrated. The chamber was flooded with CO2, leading to the dissolution of CO2 and acidification of the thin water layer. In effect, an acidic surface electrolyte formed which gave rise to a higher surface conductivity and induced a more rapid conversion of ZnO to ZnCO3.20,32 The presence of acidified surface water on our quartz coverslip produced greater affinity for ZnCO3 to grow laterally along the surface.13 The lateral conversion to ZnCO3 must be accompanied, however, with some vertical growth because for the 0 day exposure the NW showed more SHG enhancement compared to the NW after 6 days of exposure (see below). If there was no vertical conversion of ZnO to the carbonate form, the central wire enhancement for 0 and 6 days should have been comparable. The enhancement effect depends upon a close approach of the metal tip to the SHG active material, typically less than 10 nm (see below). The observation that the vertical growth is slow in regions that showed substantial lateral growth may explain why enhancement was still observed there. The enhancement, however, decreased with respect to exposure time. The decreased height in Box 1 is depicted in the sketch shown in Figure 3E. In Box 2 of Figure 4A, the NW height ranges from 55 to 110 nm. This larger, but still reduced, height is an indication that the wire had also started to degrade in this region. Referring to Figure 3B and 3F, the growth of ZnCO3 on the surface of the ZnO NW and the etching of a ZnO NW can both occur. Thus, the uneven topography and SHG signal distribution along the NW length can be explained. Finally, in Box 3 of Figure 4A, the NW height ranges from 110 to 220 nm. This indicates that the corrosion in this region resulted in vertical growth of zinc carbonate. The lower density of ZnCO3 relative to ZnO results in an expanded volume. As illustrated by

Figure 3C and 3D, an asymmetric growth of ZnCO3 can occur which helps explain the varying height along the NW. Figure 4B is an NF image of the NW after two days of exposure that shows that the SHG enhancement varied along the NW similarly, but inversely, to the AFM height data. The increased SHG enhancement in Box 4 of Figure 4B relative to the remainder of the wire corresponds to a decreased NW height in Figure 4A. A dashed line indicating the nanowire AFM outline (derived from Figure 4A) is shown for clarity. The SHG enhancement was more pronounced in Box 4 of Figure 4B because in this region much of the ZnO NW was converted laterally to zinc carbonate. Thus, the flattening and etching of the NW rendered the remainder of the uncorroded ZnO NW closer in distance to the gold-coated AFM tip. This lateral growth effect is illustrated in Figure 3E. Tip enhancement is a short-range effect calculated to extend only on the order of 10 nm from the tip apex.27 Enhancement was pronounced as the corroded NW local height decreased because the lateral growth of ZnCO3 did not greatly affect the tip−ZnO distance. In other regions along the nanowire, zinc carbonate formation on the top surface of the NW held the AFM tip further away from the remainder of the ZnO NW core. At areas along the NW length that were not entirely covered with ZnCO3 because the product material spread laterally, the remaining ZnO NW core could corrode further. A thinner carbonate layer promoted greater tip enhancement. Comparison of the NF SHG image (Figure 4B) to the AFM image (Figure 4A) shows that the height of the NW varied from low to high over Boxes 1, 2, and 3; this trend corresponds to changes in the enhancement in Figure 4B from more to less. For lower topographic height, more SHG signal enhancement was observed. Box 5 of Figure 4B shows a “ghost image” of Box 10409

dx.doi.org/10.1021/jp301922a | J. Phys. Chem. C 2012, 116, 10405−10414

The Journal of Physical Chemistry C

Article

Figure 4. AFM, tip-enhanced, near-field and far-field SHG images of a bare ZnO nanowire after exposure to carbon dioxide and water vapor. (A−C) 2 day exposure; D−F) 4 day exposure; and (G−I) 6 day exposure. Note that for clarity an AFM NW outline (dashed line, ----) was reproduced on the near-field SHG images. The color bar units and offsets are described in the Figure 2 caption.

4. The reason for this artifact is unknown, but similar effects were observed in other images and are not reproducible. The ghosts could be due to tip diffractive effects for certain optical alignments. An FF-SHG image of the exposed NW is shown in Figure 4C; comparing the Figure 4C FF-SHG image to that of Figure 2C, the two-day exposed NW FF-SHG signal is lower over the full length of the NW, as is expected if the oxide is being consumed in the corrosion. Because the electric field enhancement effect only extends a few nanometers from the metal tip, our enhanced NF-SHG signal is derived only from a thin outer shell of the NW; the FF-SHG signal is due to the entire bulk of the NW. Thus, comparison of NF and FF images is difficult for surface-reacting NWs. Further, the FF images lack sufficient spatial resolution to discern nanometer length scale features. Figure 4D−F shows images of the same zinc oxide NW exposed to carbon dioxide and water vapor for four days. In Figure 4D Box 6 has a height ranging from approximately 43 to 63 nm NW height; Box 7 encloses an area with a NW height from 63 to 106 nm; and Box 8 is an area with height from 63 to 170 nm. The conversion of the ZnO NW to zinc carbonate was not uniform along the nanowire, similar to a combination of

Figure 3C and 3D depictions. Carbonate formation changed from lateral to vertical growth along the length of the NW. Figure 4E is an NF-SHG image corresponding to the AFM image (Figure 4D). Noticeably, there is more SHG signal enhancement in Box 9 of Figure 4E since the NW conversion to zinc carbonate is lateral; the gold-coated AFM tip probed closer to the ZnO core thus inducing more signal enhancement than for Box 10. Box 10 of Figure 4E does not show a pronounced enhancement since the SHG-inactive zinc carbonate is growing vertically, as seen in Figure 4D (Box 8). Thus, the thicker ZnCO3 layer prevents the probe tip from approaching the ZnO core. The decrease in FF-SHG intensity observed in Figure 4F is explained by formation of SHGinactive zinc carbonate. Figure 4 G−I shows images of the same NW after six days of exposure. The apparent 0.4 μm vertical shift along the image yaxis is due to our limited ability to relocate the AFM tip precisely to its “home” position relative to the NW. However, the AFM and NF-SHG images are necessarily in registration because both data acquisitions share the same tip raster. Figure 4G shows a topographic image of the 6 day exposed zinc oxide NW. Box 11 of Figure 4G encompasses heights that range from 33 to 81 nm, while Box 12 has a range of heights from 16 to 33 10410

dx.doi.org/10.1021/jp301922a | J. Phys. Chem. C 2012, 116, 10405−10414

The Journal of Physical Chemistry C

Article

Figure 5. AFM, tip-enhanced, near-field and far-field SHG images of two different 3 nm alumina-coated ZnO nanowires before and after exposure to carbon dioxide and water vapor for (A−C) 0 day exposure and (D−F) 6 day exposure. Note that the NWs used for data collection for A−C and D− F are not the same. For clarity, an AFM NW outline (dashed line, ----) was reproduced on the near-field SHG images.

Table 1. Summary of Image Data for the Uncoated ZnO NWs (Figures 2 and 4) AFM

NF-SHG

FF-SHG

exposure time (days)

height range (nm)

relative intensity range (au)

average NW pixel intensity (au)

0 (Figure 2 A−C) 2 (Figure 4 A−C)

60−95

4 (Figure 4 D−F)

6 (Figure 4 G−I)

Box Box Box Box Box Box Box Box Box

1 2 3 6 7 8 11 12 13

55−83 55−110 110−220 43−63 63−106 63−170 33−81 16−33 47−130

Box 4

1.1 × 105−2.2 × 105 ∼7.0 × 104−1.05 × 105

7334 4412

Box 5 Box 9

∼7.0 × 104−1.05 × 105 3.8 × 104−1.50 × 105

650

Box Box Box Box

3.8 2.6 2.6 1.8

10 14 15 16

× × × ×

104−5.6 104−3.5 104−7.0 104−2.6

× × × ×

104 104 104 104

509

vapor. Morphological degradation of the NW was observed, and the SHG enhancement varied in regions along the NW. There are several possible forms for zinc carbonate growth, as sketched in Figure 3. When the formation of the ZnCO3 film resulted in an increase in NW height, minimal SHG enhancement was observed. On the other hand, the height of the NW decreased when the zinc carbonate formation led to lateral growth. Then the upper extent of the relatively bare ZnO NW core led to an increase in enhancement compared to other regions. The observed inhomogeneity of zinc carbonate formation along the zinc oxide NW may occur because at some regions the zinc carbonate is not compact, and thus the inner zinc oxide NW core can continuously corrode. Such a phenomenon may result from the formation of pendant zinc carbonate NWs and nanorods, while the ZnO core is continuously etched away. This is a plausible explanation for the nonuniformity observed, some flattening and some increased height at different NW regions.13 Once an increase in width or flattening occurs, the

nm. Since the image is shifted, Box 12 (Figure 4G) is comparable to Box 6 of Figure 4D. Then, Box 13 (Figure 4G) has height values ranging from 47 to 130 nm which is comparable with the Box 7 and 8 regions of Figure 4D. As explained using previous figure panels, Box 14 of the Figure 4H NF-SHG image shows a minimal enhancement since, from Figure 4G, this region has a height comparable to the initial NW. Box 15 of Figure 4H showed more enhancement compared to regions corresponding to Boxes 14 and 16. By comparing to Box 12 of Figure 4G, the height is smaller (most of the ZnO NW is converted to zinc carbonate by growing unevenly and laterally and exposing more of the core of the ZnO NW). Thus, the enhancement in the SHG signal increased because the AFM probe is not prevented by zinc carbonate from approaching the ZnO NW core. Figure 4I is an FF-SHG image of the NW, and as observed earlier, the SHG intensity was further decreased relative to the starting NW. In general, corrosion of the bare ZnO NW started immediately after exposure to carbon dioxide and water 10411

dx.doi.org/10.1021/jp301922a | J. Phys. Chem. C 2012, 116, 10405−10414

The Journal of Physical Chemistry C

Article

Table 2. Summary of Image Data Collected from Two Alumina-Coated ZnO NWs (Figure 5) exposure time (days) 0 (Figure 5 A−C) 6 (Figure 5 D−F)

AFM

NF-SHG

FF-SHG

height range (nm)

relative intensity range (au)

average NW pixel intensity (au)

60−120 Box 1 Box 2

50−100 50−70

Box 3 Box 4

6.5 × 105−2.8 × 105 ∼4.8 × 104−1.2 × 105 ∼7.1 × 104−1.9 × 105

3239 1319

Figure 6. Comparison of AFM, tip-enhanced, near-field and far-field images for fresh (A−C) 0 nm; (D−F) 3 nm, and (G−I) 6 nm alumina-coated ZnO nanowires.

image of the NW was recorded and is shown in Figure 5B. A varying SHG signal enhancement is evident along the NW. The inhomogeneity may be due to the following reasons: (a) small thickness variation of the alumina along the ZnO NW, (b) nonuniformity of the laser beam profile, or (c) laser instability during the 2 h data acquisition. NW degradation should be minimal since the fresh zinc oxide NW was coated with alumina immediately before imaging. Figure 5C illustrates the FF-SHG image of the alumina-coated zinc oxide nanowire. The SHG nonuniformity along the NW length is possibly due to inhomogeneity of the 800 nm beam profile. Figures 5D−F are images of a dif ferent alumina-coated NW after 6 days of exposure. Figure 5D is a topographical profile of the alumina-coated NW. Box 1 of Figure 5D shows height values within a 50−100 nm range, and the Box 2 region shows height values from approximately 50 to 70 nm. A plausible

remainder of the ZnO NW core can be more directly probed by the gold-coated tip allowing for greater SHG signal enhancement. The distance of the tip apex from the surface of the active sample material dictates the SHG signal enhancement magnitude. Figure 5 shows NF- and FF-SHG images of an aluminacoated zinc oxide NW. The atomic layer deposition (ALD) alumina coating was approximately 3 nm thick and is ostensibly amorphous. Alumina was used to test the protection capability of an inert shell layer to prevent ZnO corrosion. The exposure of the alumina-coated ZnO NW was performed in the same chamber and at the same time as the bare ZnO NW. Figures 5A−C show a single alumina-coated ZnO NW with no CO2/ H2O exposure. Figure 5 image data are summarized in Table 2. Figure 5A is a topographical image of the NW where the height ranges from 75 to 100 nm. A tip-enhanced nonlinear NF-SHG 10412

dx.doi.org/10.1021/jp301922a | J. Phys. Chem. C 2012, 116, 10405−10414

The Journal of Physical Chemistry C

Article

nanometer coatings due to the diffraction limit. However, characterizing the NWs with the tip-enhancement method where a few nanometer scale approach distance is required distinguishes different coating thicknesses.

scenario for an explanation of images 5D and 5E is as follows. The NW was initially equal height and uniform along its length as was observed for all the NWs described here, including that in 5A. The height data of Figure 5D indicates that vertical growth occurred at the lower half of the NW (Box 1 of Figure 5D), where presumably the Al2O3 coating failed. Then, the NFSHG image in 5E is appropriate, as where the carbonate formed, the scanning tip was blocked from closely approaching the ZnO wire and thus yielded lower signal enhancement. Figure 5F is an FF-SHG image of the exposed alumina-coated NW. Only partial degradation of the alumina-coated NW was observed, and so the alumina coating was able to somewhat protect the NW from exposure to carbon dioxide and water vapor over a period of 6 days. A possible degradation of the alumina coating might be due to water adsorption on its surface. The presence of water adsorbed on the surface of oxides such as alumina occurs in different forms (molecular via hydrogen and donor−acceptor bonds, or dissociative adsorption, or in clusters of different sizes) and has a significant influence on oxide surface properties.31 Alumina surface interactions with water may lead to the dissolution of alumina, induced by the adsorption of H+ ions.33 Comparing the topographical profiles of coated and bare NWs after 6 days of exposure, the alumina-coated zinc oxide NW was substantially more intact. For a high water partial pressure, as in our experiment, the surface of the alumina was substantially hydrated and thus exhibited a reduced ability to protect the zinc oxide. Six days of exposure was not enough to completely hydrate the alumina; thus, only a small amount of corrosion was observed. Figure 6 compares the AFM, tip-enhanced NF- and FF-SHG images of unexposed alumina-coated wires with 0, 3, and 6 nm coating thickness. Figures 6A, D, and G present topographic profiles of the bare and coated NWs; comparing the three images does not produce any noteworthy differences except that they have slightly different height values. Comparing Figures 6B, E, and H, the NF-SHG enhancement clearly decreases as the coating thickness increases, and is evidence that the enhancing metal tip must closely approach the SHGactive sample material in order for the amplified electric field magnitude to have much effect. Since the alumina-coated NW in Figure 6G−I has a 6 nm thickness, minimal NF signal enhancement was observed. On the basis of theoretical calculations, the minimum distance between the gold-coated AFM tip and the surface of the active material must be less than approximately 10 nm to observe strong second harmonic signal enhancement.27 A 3 nm layer of alumina was thick enough to partially protect or delay the corrosion process of the zinc oxide NW but still allowed for NW characterization using the tip-enhanced nonlinear NF imaging technique. Thus, this method is an excellent choice when few nanometer vertical selectivity is needed for characterization of nanotextured samples. Further, the NW NF-SHG signals recorded throughout this study must largely be due to the outer shell of the bulk NW active material, as opposed to only an interfacial SHG signal. Otherwise the amorphous coating would still have produced an enhanced SHG photon yield due to the symmetry break at its air interface in conjunction with a NF tip. Figures 6C, F, and I are FF-SHG images of the aluminacoated NWs; not much difference is observed since the images do not display sufficient spatial resolution to discern few

■

CONCLUSIONS A nonlinear microscopy technique was used to record subdiffraction-limited spatial resolution images showing localized corrosion effects of a single ZnO nanowire over time. Spatial resolution approaching 100 nm was attained using apertureless NF optical second-harmonic generation imaging. Complementary topographic AFM data for physical imaging were created during the same sample scan. In some regions of the NW, the zinc carbonate corrosion product grew vertically which reduced the enhanced SHG signal in that particular region since zinc carbonate is SHG inactive, and further, it held the gold-coated probe tip at too great a distance from the zinc oxide NW core for effective optical field magnification. Some areas on the same NW, on the other hand, showed formation of zinc carbonate growing laterally. There, the remaining portion of the ZnO NW was subject to closer approach by the gold-coated AFM tip, resulting in greater enhancement. Thus, the tip-enhanced SHG microscopy method was used to image formation of submicrometer corrosion. For a FF-SHG microscopy method, a minute layer of zinc carbonate would not be evident because the core of the bulk NW still yields 400 nm photons. FF images only displayed a uniform chemistry, while the NF images showed nonuniformity along the NW length, providing contrast between different corrosion morphologies. Bare zinc oxide NWs degraded faster compared with alumina-coated NWs upon exposure to carbon dioxide and water vapor. A 3 nm alumina-coated zinc oxide NW was more intact, in terms of its form, after exposure for 6 days, as compared to a bare zinc oxide NW. Alumina delayed the corrosion process but may not be the optimum inorganic coating. The NF-SHG imaging technique is useful for detecting small, corroded regions on a nanoparticle. One limitation of this instrument is that the sample material must be SHG active; however, an odd order harmonic generation (e.g., third harmonic generation (THG)) technique22 also coupled with metalized AFM tips is under investigation. The THG nonlinear process is symmetry-allowed for a wider selection of samples. Thus, different semiconductor materials can be analyzed, for example, silicon NWs.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: 865-574-4920. Fax: 865-574-8363. E-mail: shawrw@ornl. gov. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy. The authors thank W. B. Whitten and Y.-Z. Ma for valuable comments. 10413

dx.doi.org/10.1021/jp301922a | J. Phys. Chem. C 2012, 116, 10405−10414

The Journal of Physical Chemistry C

■

Article

REFERENCES

(1) Griebner, U.; Kaindl, R. A.; Elsaesser, T.; Seeber, W. Appl. Phys. B 1998, 67, 757. (2) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R. J.; Yang, P. Nat. Mater. 2005, 4, 455. (3) Leschkies, K. S.; Divakar, R.; Basu, J.; Enache-Pommer, E.; Boercker, J. E.; Carter, C. B.; Kortshagen, U. R.; Norris, D. J.; Aydil, E. S. Nano Lett. 2007, 7, 1793. (4) Cheng, H. M.; Chiu, W. H.; Lee, C. H.; Tsai, S. Y.; Hsieh, W. F. J. Phys. Chem. C 2008, 112, 16359. (5) Bao, J. M.; Zimmler, M. A.; Capasso, F.; Wang, X. W.; Ren, Z. F. Nano Lett. 2006, 6, 1719. (6) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (7) Johnson, J. C.; Yan, H.; Schaller, R. D.; Haber, L. H.; Saykally, R. J.; Yang, P. J. Phys. Chem. B 2001, 105, 11387. (8) Arnold, M. S.; Avouris, P.; Pan, Z. W.; Wand, Z. L. J. Phys. Chem. B 2003, 107, 659. (9) Pearton, S. J.; Norton, D. P.; Ren, F. Small 2007, 3, 1144. (10) Law, J. B. K.; Thong, J. T. L. Appl. Phys. Lett. 2006, 88, 133114. (11) Klingshirn, C. ChemPhysChem 2007, 8, 782. (12) Klingshirn, C. Phys. Status Solidi B 2007, 244, 3027. (13) Pan, Z.; Tao, J.; Zhu, Y.; Huang, J.-F.; Paranthaman, M. P. Chem. Mater. 2010, 22, 149. (14) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. Adv. Funct. Mater. 2003, 13, 9. (15) Huang, M. H.; Wu, Y. Y.; Feick, H.; Tran, N.; Weber, E.; Yang, P. D. Adv. Mater. 2001, 13, 113. (16) Ye, C. H.; Fang, X. S.; Hao, Y. F.; Teng, X. M.; Zhang, L. D. J. Phys. Chem. B 2005, 109, 19758. (17) Yang, P. D.; Yan, H. Q.; Mao, S.; Russo, R.; Johnson, J.; Saykally, R.; Morris, N.; Pham, J.; He, R. R.; Choi, H. J. Adv. Funct. Mater. 2002, 12, 323. (18) Xia, Y. N.; Yang, P. D. Adv. Mater. 2003, 15, 351. (19) Falk, T.; Svensson, J. E.; Johansson, L. G. J. Electrochem. Soc. 1998, 145, 39. (20) Falk, T.; Svensson, J. E.; Johansson, L. G. J. Electrochem. Soc. 1998, 145, 2993. (21) Yang, Y.; Kim, D. S.; Qin, Y.; Berger, A.; Scholz, R.; Kim, H.; Knez, M.; Gosele, U. J. Am. Chem. Soc. 2009, 131, 13920. (22) Schaller, R. D.; Johnson, J. C.; Wilson, K. R.; Lee, L. F.; Haber, L. H.; Saykally, R. J. J. Phys. Chem. B 2002, 106, 5143. (23) Johnson, J. C.; Yan, H.; Schaller, R. D.; Petersen, P. B.; Yang, P.; Saykally, R. J. Nano Lett. 2002, 2, 279. (24) Fitts, J. P.; Shang, X. M.; Flynn, G. W.; Heinz, T. F.; Eisenthal, K. B. J. Phys. Chem. B 2005, 109, 7981. (25) Cimatu, K.; Baldelli, S. J. Am. Chem. Soc. 2008, 130, 8030. (26) Meyer, K. A.; Ng, K. C.; Gu, Z.; Pan, Z.; Whitten, W. B.; Shaw, R. W. Appl. Spectrosc. 2010, 64, 1. (27) Hartschuh, A.; Beversluis, M. R.; Bouhelier, A.; Novotny, L. Philos. Trans. R. Soc. London, A 2004, 362, 807. (28) Xing, Y. J.; Xi, Z. H.; Zhang, X. D.; Song, J. H.; Wang, R. M.; Xu, J.; Xue, Z. Q.; Yu, D. P. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 1527. (29) Mahurin, S.; Bao, L. L.; Yan, W. F.; Liang, C. D.; Dai, S. J. NonCryst. Solids 2006, 352, 3280. (30) Mahurin, S.; John, J.; Sepaniak, M.; Dai, S. Appl. Spectrosc. 2011, 65, 417. (31) Gunko, V. M.; Turov, V. V.; Zarko, V. I.; Voronin, E. F.; Tischenko, V. A.; Dudnik, V. V.; Pakhlov, E. M.; Chuiko, A. A. Langmuir 1997, 13, 1529. (32) Lindstrom, R.; Svensson, J. E.; Johansson, L. G. J. Electrochem. Soc. 2000, 147, 1751. (33) Trueba, M.; Trasatti, S. P. Eur. J. Inorg. Chem. 2005, 17, 3393.

10414

dx.doi.org/10.1021/jp301922a | J. Phys. Chem. C 2012, 116, 10405−10414