pubs.acs.org/NanoLett
Catalyst-Free Synthesis and Structural and Mechanical Characterization of Single Crystalline Ca2B2O5·H2O Nanobelts and Stacking Faulted Ca2B2O5 Nanogrooves Lihong Bao, Zhi-Hui Xu, Rui Li, and Xiaodong Li* Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Columbia, South Carolina 29208 ABSTRACT Single crystalline Ca2B2O5·H2O (CBOH) nanobelts were synthesized for the first time using a facile catalyst-free hydrothermal method at low temperature. After being annealed at 400 °C for 10 h, the CBOH nanobelts transformed into curveshaped Ca2B2O5(CBO) nanogrooves with ordered stacking fault arrays along the [1¯10] direction. The single crystalline CBOH nanobelts and the stacking faulted CBO nanogrooves provided a unique platform for investigating the effects of hydrated H2O and stacking faults on the mechanical properties of nanomaterials. Nanoindentation tests were performed directly on individual CBOH nanobelts and CBO nanogrooves to probe their mechanical properties. It was found that the CBOH nanobelts, with the presence of hydrated H2O, achieved 28.7% increase in elastic modulus, whereas the stacking faulted CBO nanogrooves, with the absence of hydrated H2O, exhibited 99% loss in elastic modulus, compared to bulk CBO. KEYWORDS Nanobelts, nanogrooves, stacking faults, nanoindentation, elastic modulus
O
(UV) light sources, light emission diodes (LEDs), and luminescent phosphors.30-34 Several techniques have been developed to synthesize bulk calcium borate materials, such as solid state reactions between boric acid (boron oxide) and calcium oxide (carbonates) and hydrothermal and microemulsion methods. However, to our knowledge, calcium borate nanostructures are still absent in literature. Liu et al.35 reported a general composite-hydroxide-mediated approach to synthesizing complex functionalized oxide nanostructures. In the present study, we successfully synthesized single crystalline Ca2B2O5·H2O (CBOH) nanobelts for the first time using a facile catalyst-free hydrothermal method at low temperature. The hydrated H2O in the CBOH nanobelts was completely removed by annealing the CBOH nanobelts at 400 °C, and consequently the CBOH nanobelts transformed into curve-shaped Ca2B2O5 (CBO) nanogrooves. Ordered stacking fault arrays along the [1¯10] direction were found in these CBO nanogrooves. Nanoindentation tests performed on individual CBOH nanobelts and CBO nanogrooves revealed that the CBOH nanobelts, with the presence of hydrated H2O, achieved 28.7% increase in elastic modulus, whereas the stacking faulted CBO nanogrooves, with the absence of hydrated H2O, exhibited 99% decrease in elastic modulus, compared to bulk CBO. These findings are significant for tailoring the mechanical properties of nanomaterials by controlling their compositions and microstructure. CBOH nanobelts were synthesized by a facile catalystfree hydrothermal method. In a typical experiment, 2 mmol of CaCl2 (anhydrous, 99%, Alfa Aesar), 6 mmol of H3BO3 (99.0%, Fisher Corp.), and 15 mmol of NaOH (99%, Fisher Corp.) were dissolved in 15 mL of distilled water with
ne dimensional (1D) nanostructures, such as nanotubes, nanowires, and nanobelts, have attracted tremendous attention due to their intriguing properties and potential applications in electronics, photonics, and mechanics.1-6 Nanotechnology provides a unique platform for tailoring the physical properties of nanomaterials by controlling their compositions and microstructure.7-11 Defects, such as stacking faults, twins, and dislocations, not only are essential for the growth of nanostructures but also strongly influence their mechanical, optical, and electrical properties.12-22 Stacking faults, which are commonly found in wurtzite and zinc blende III-V semiconductors,12-16 have a great influence on the optical properties of nanostructures14 and may lead to the malfunction or even failure of future nanoelectronic devices due to the electron scattering at stacking faults.23,24 Recently, ordered stacking faults were observed in Si nanowires.25 Twins were found in magnesium borate nanowires.22 To date, however, stacking faults have not been discovered in metal borate nanostructures yet. Metal borates, such as magnesium borate, aluminum borate, and barium borate, are remarkable ceramic materials with excellent mechanical properties, good chemical inertness, high stability under high temperature, and lightweight, and low thermal expansion coefficients.22,26-29 Calcium borate has a great potential in the applications for glass fibers, ceramic coatings, dielectric devices, ultraviolet
* To whom correspondence should be addressed,
[email protected]. Received for review: 10/13/2009 Published on Web: 12/03/2009 © 2010 American Chemical Society
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DOI: 10.1021/nl9034187 | Nano Lett. 2010, 10, 255-262
FIGURE 1. XRD, SEM, TEM, and AFM characterization of Ca2B2O5·H2O (CBOH) nanobelts. (a) XRD spectrum of the products synthesized at 180 °C for 12 h. (b) Low-magnification SEM image of CBOH nanobelts. (c) High-magnification SEM image, inset is the SEM image of a single nanobelt. (d) Bright-field TEM image of CBOH nanobelts. (e) TEM image of a single CBOH nanobelt, inset is its corresponding SAED pattern. (f) HRTEM image of a CBOH nanobelt, inset is its FFT pattern. (g) Cross-sectional TEM images of a single CBOH nanobelt, right above inset is the corresponding SAED pattern and the left below inset is a schematic. (h) 3D AFM image and (i) section analysis profile of a single CBOH nanobelt, showing that the nanobelt has a thickness of 30 nm.
constant magnetic stirring at room temperature. The solution turned turbid immediately, and a great amount of suspended precipitates were formed. Then the suspension was transferred into a 23 mL Teflon-lined stainless steel autoclave, and the autoclave was maintained at 180 °C for 8 h. After cooling down to room temperature, the obtained precipitates were washed, filtered, and dried to obtain the final product. The as-prepared powders were analyzed by X-ray diffraction (XRD, Rigaku DMax 2200 using Cu KR radiation, λ ) 1.5418 Å), scanning electron microscopy (SEM, Quanta 200, FEI Corp.) and energy-dispersed X-ray spectroscopy (EDX). A few drops of ethanol solution containing nanobelts were dropped onto copper grids or Si substrate for transmission electron microscopy (TEM, Hitachi H-8000), selected area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM, JEOL JEM2010F) and nanoindentation (Hysitron, Triboscope nanoindenter) studies. Crosssectional TEM specimens were prepared by slicing nanobelts embedded in Spurr’s Epoxy resin with a Sorvall Porter-Blum MT2-B ultramicrotome. The Hysitron triboscope nanoindenter in conjunction with the Veeco Dimension 3100 atomic force microscope was used to perform imaging and nanoindentation tests. The nanoindenter monitored and recorded the load and displacement of the three-sided pyramidal diamond (Berkovich) © 2010 American Chemical Society
indenter during indentation with a force resolution of about 1 nN and displacement resolution of about 0.2 nm.36,37 The indenter tip was used to image and locate a single CBO nanobelt and then in situ indent the belt with the same tip. The indentation mark was also imaged with the same tip. Post-test imaging provides the ability to verify that the test was performed in the anticipated location, which maximizes the reliability of data and aids in explanation of unexpected test results. Hardness and elastic modulus were calculated from the load-displacement data obtained by nanoindentation. Before each nanoindentation test, the thermal drift was automatically tracked and recorded by means of introducing the nanoindenter in touch with the top surface of the sample with minimum contact load. All the nanoindentation tests were performed when the thermal drift or vibration induced mechanical drift rate dropped down to 0.01 nm/s. The load-displacement curves were obtained by subtracting the drift effect for hardness and elastic modulus analyses. Figure 1a shows the XRD spectrum of the as-prepared product. All peaks can be indexed into the monoclinic phase of Ca2B2O5·H2O (CBOH) (JCPDS No. 22-0145, a ) 0.6719 nm, b ) 0.5434 nm, c ) 0.3555 nm, R ) γ ) 90°, β ) 92.87°). No diffraction peaks of impurity phases such as CaO and B2O3 were detected in the XRD spectrum, indicating high purity of the product. The SEM (parts b and c of Figure 1) and TEM (parts d and e of Figure 1) images show that the 256
DOI: 10.1021/nl9034187 | Nano Lett. 2010, 10, 255-262
FIGURE 2. Growth processes of the as-synthesized Ca2B2O5·H2O (CBOH) nanobelts. (a-d) SEM images and (e, h) corresponding EDAX spectra of the products under different experimental conditions. (a) and (e) Precursor without heating. (b) and (f) After heating at 180 °C for 1 h. (c) and (g) After heating at 180 °C for 3 h. (d) and (h) After heating at 180 °C for 8 h. (i) Schematic of the growth processes of the CBOH nanobelts.
product consists of abundant straight CBOH nanobelts with widths ranging from 100 to 500 nm and lengths up to tens of micrometers. The nanobelts have very smooth surfaces and uniform sizes along their longitudinal directions. The SAED pattern in Figure 1e can be indexed as single crystalline monoclinic structure of Ca2B2O5·H2O with the [100] zone axis. The HRTEM image (Figure 1f) reveals that the CBOH nanobelt is single crystalline, and the growth direction of the CBOH nanobelt is along the [104] direction. The crosssectional TEM image of the CBOH nanobelt (Figure 1g) shows an intriguing hexagonal structure including two anisotropic large facets and four anisotropic small facets, suggesting high anisotropy in the lateral crystal growth of the nanobelt. The three-dimensional (3D) atomic force microscopy (AFM) image and corresponding section analysis profile of a representative CBOH nanobelt (parts h and i of Figure 1) reveal that the CBOH nanobelt has a smooth cross section along its length with thicknesses ranging from 20 to 50 nm. © 2010 American Chemical Society
One of the characteristics of the 1D nanostructures synthesized via vapor-liquid-solid (VLS)38 or solutionliquid-solid (SLS)39 growth mechanism is that the obtained nanostructures have catalytic nanoparticles terminated at the ends of the nanowires or nanobelts. From the SEM and TEM results (shown in Figure 1), clearly no catalytic nanoparticles were observed at the ends of the CBOH nanobelts. Therefore, the growth mechanism of CBOH nanobelts is neither VLS nor SLS. To seek an in-depth understanding of the growth mechanism of the as-synthesized CBOH nanobelts, a series of systematic experiments were carried out to investigate the effects of precursors, reaction temperature, and reaction time on the CBOH nanobelt formation. We found that NaOH played a critical role in the synthesis process. Without NaOH in the precursor, only small amounts of boron oxide solid products were obtained. The SEM image (Figure 2a) and corresponding EDX spectrum (Figure 2e) reveal that after adding NaOH in the precursor, without 257
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heating only CaO nanoparticles formed. In the heat treatment at 180 °C, CBOH nanobelts were formed with the increase of yield rate as a function of time (parts b-d of Figure 2). In our experiments, Ca2+ and OH- were provided by CaCl2 and NaOH. When the precursor solution was heated to 180 °C, hydrothermal reactions occurred as follows
Ca2+ + 2OH- f Ca(OH)2
after annealing at 400 °C for 10 h the CBOH transformed into CBO with a complete removal of H2O. The SEM images (Figure 3b,c) and TEM image (Figure 3d) reveal that the CBO has a groovelike morphology. The CBO nanogrooves are straight but have rough surfaces. Vacancy zones were formed in the nanogrooves due to evaporation of the H2O molecules and reconstruction of the crystal lattices. The SAED pattern in the inset of Figure 3d reveals that the CBO nanogrooves are single crystalline and can be indexed as the monoclinic Ca2B2O5 single crystal with the [112] zone axis. The cross-sectional TEM image (Figure 3e) shows that the CBO nanogroove has an irregular curved hexagonal shape. The HRTEM image (Figure 3f) reveals stacking faults in the CBO nanogrooves. The spacing between these stacking fault arrays was measured to be 0.70 nm, which is 3/2 times that of the d-spacing of the (1¯10) plane. Figure 3g shows the SAED pattern of the nanogroove, and the enlarged pattern below exhibits the diffraction intensity along the [1¯10] direction. The light blue circles can be indexed as (1¯10)and (11¯0) spots, which come from the nonstacking fault (normal crystalline lattice) region. The green circles, which result from the stacking fault arrays, can be indexed as 3/2{1¯10} spots. This suggests that the perfect stacking sequence of monoclinic structure as ...ABAB... along the [110] direction transformed into ...ABCABC.... Figure 3h shows the HRTEM image of the interface between the stacking faults and the normal crystalline lattice of the nanogroove, where orange frame and blue frame are corresponding magnified images. The orange frame shows the interface lattices whereas the blue frame exhibits the normal crystalline lattices, further confirming the presence of 3/2{1¯10} stacking faults in the CBO nanogroove. Figure 3i is the HRTEM image showing the structural details of the vacancy zones formed in the CBO nanogrooves, indicated by white arrows. No lattice mismatch was found between the vacancy zones and the surrounding structures. The 3D AFM image and section analysis profile of individual CBO nanogrooves in parts j and k of Figure 3 further confirm the groove-shape characteristic. The CBOH nanobelts and CBO nanogrooves were dispersed into ethanol solution, and then a few drops of solution were dropped onto a clean Si substrate for the nanoindentation experiments. A Berkovich diamond nanoindenter tip was used to image and locate a single CBOH nanobelt or CBO nanogroove and then in situ to position the tip on the belt/groove to perform an indentation test. The peak nanoindentation depth can be as low as 13 nm, which is about 20% of the belt thickness. It is generally accepted that the depth of indentation should never exceed 30% of the belt thickness (film thickness) to minimize the substrate effect on the measurements of the hardness and elastic modulus of the belt.36,37,42 The indentation mark was imaged immediately after the indentation test using the same tip. The indentation projected area obtained by the AFM was used to calibrate the hardness and elastic modulus values.
(1)
180 °C
2Ca(OH)2 + 2H3BO3 98 Ca2B2O5·H2OV + 4H2O (2) Figure 2i schematically illustrates the growth processes of the CBOH nanobelts. At the early stage of growth, due to the low solubility of Ca(OH)2 in water, Ca2+ reacted immediately with OH- to form suspended Ca(OH)2 precipitates in the precursor solution. The heat treatment at 180 °C further lowered the solubility of Ca(OH)2, resulting in more precipitates. The Ca(OH)2 precipitates offered energy-favored sites for the nucleation and growth of CBOH nanobelts. While H3BO3, which was dissolved in the aqueous solution, reacted with Ca(OH)2 to form CBOH at the solidliquid (S-L) interface. Once the concentration of CBOH reached the supersaturation state, CBOH crystal nuclei formed at the liquid-solid (L-S) interface. The nuclei aggregated together through intermolecular van der Waals force to form relatively larger structures, such as nanoclusters or nanocrystals. In general, in order to lower the surface energy, nanocrystals tended to agglomerate to decrease their exposed surfaces. The same types of crystal planes then aligned with each other, and a coherent interface formed to minimize the interface stain energy.40 Therefore, the aggregated nuclei (nanoclusters or nanocrystals) attached to each other along the direction of fast growth, and finally onedimensional (1D) growth formed due to the anisotropic nature of CBOH crystal. In 1D growth, the degree of supersaturation plays a critical role in crystal nucleation and crystal growth rate, which determines the prevailing growth morphology.41 In the whole hydrothermal process, the concentration of calcium decreased with growth time owing to the formation of CBOH precipitates. However, the PH value of the solution nearly kept constant because excessive sodium hydroxide was used. On one side, excessive NaOH, namely, with high concentration of OH-, ensured to offer sufficient driven forces for the precipitation reactions to generate CBOH product. On the other hand, a high concentration of OH- led to a high degree of supersaturation, which resulted in a high crystal nucleation and growth rate. Under these conditions, lateral growth was restricted and 1D CBOH nanoblets were formed. Figure 3a shows the XRD spectrum of the annealed CBOH nanobelts after annealing at 400 °C for 10 h in air. All peaks can be indexed into the monoclinic phase of Ca2B2O5 (CBO) (JCPDS No. 22-0139, a ) 1.149 nm, b ) 0.5157 nm, c ) 0.7200 nm, R ) γ ) 90°, β ) 92.91°). This suggests that © 2010 American Chemical Society
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FIGURE 3. XRD, SEM, TEM, and AFM characterization of Ca2B2O5 (CBO) nanogrooves: (a) XRD spectrum of CBO nanogrooves. (b) and (c) SEM images of CBO nanogrooves. (d) Bright-field TEM image of a CBO nanogroove, inset is its corresponding SAED pattern. (e) Cross-sectional TEM image of a CBO nanogroove. (f) HRTEM image of a CBO nanogroove showing ordered stacking fault arrays. (g) SAED pattern of the CBO nanogroove, the figure below is the enlarged portion of the SAED pattern showing nonzero intensity at (3/2){1¯10} positions. (h) HRTEM image showing the interface between the stacking faults and the normal lattices of the CBO nanogrooves, orange and blue frames are corresponding enlarged regions, where dashed lines show the fringes of the stacking faults. (i) HRTEM image showing the vacancy zones formed on the CBO nanogrooves. (j) 3D AFM image and (k) corresponding section analysis profile of a CBO nanogroove, inset in (j) is a schematic of a nanogroove.
Figure 4 shows the 3D AFM image and corresponding section analysis profile of an indentation mark made on a single CBOH nanobelt, a representative load-displacement curve, and measured elastic modulus and hardness as a function of indentation contact depth. No cracks can be found in the area surrounding the indentation mark (Figure 4a). The measured elastic modulus and hardness of the assynthesized CBOH nanobelts were 126.6 ( 15.2 and 6.3 ( 1.2 GPa, respectively. Compared with bulk CBO of measured elastic modulus value of 98.4 GPa and hardness value of 8.5 GPa,43 a 28.7% increase in elastic modulus and 25.9% decrease of hardness in the as-synthesized CBOH nanobelts can be found. This is probably due to the presence of hydrated H2O in CBOH, which changes the crystal parameters of CBO and makes the surface atoms packed more © 2010 American Chemical Society
constrained, thereby making the CBOH nanobelts hard to deform in the elastic regime and consequently leading to the increase of the elastic modulus. A decrease in hardness indicates that plastic yielding of the CBOH nanobelts is easier than the bulk Ca2B2O5. This in turn suggests that the H2O molecules help dislocation motion in the CBOH nanobelts. Parts a and b of Figure 5 show the 3D AFM image of a single CBO nanogroove after indentation and the representative nanoinentation load-displacement curve, respectively. The cut scars by the nanoindenter were found at the CBO nanogroove edges, as shown in Figure 5a. The representative load-displacement curve (Figure 5b) reveals an almost full elastic recovery after unloading of the indenter. Figure 5c schematically shows four stages in the nanoindentation of the CBO nanogroove: (1) nanoindenter in 259
DOI: 10.1021/nl9034187 | Nano Lett. 2010, 10, 255-262
FIGURE 4. Nanoindentation on individual Ca2B2O5·H2O (CBOH) nanobelts. (a) 3D AFM image of a nanoindentation mark made on a CBOH nanobelt. (b) Cross-sectional height profile of the nanoindentation mark shown in (a). (c) Representative nanoindentation load-displacement curve. (d) Elastic modulus and hardness of as a function of indentation contact depth.
FIGURE 5. Nanoindentation on individual Ca2B2O5 (CBO) nanogrooves. (a) 3D AFM image of a CBO nanogroove after nanoindentation. (b) Corresponding nanoindentaton load-displacement curve. (c) Schematic of various stages in the nanoindentation of the CBO nanogroove.
contact with the CBO nanogroove edges; (2) extruded deformation; (3) edges buckling; and (4) recovery in unloading. In the first stage, the nanoindenter approached the CBO nanogroove and then was in contact with the nanogroove edges. In the second stage, the nanoindenter exerted an outward acting force on the CBO nanogroove edges, elastically opening the nanogroove. In the third stage, elastically opening the nanogroove encountered difficulty but the © 2010 American Chemical Society
nanoindenter kept pushing downward by cutting the nanogroove edges, leading to the edge buckling. The fourth stage started with withdrawing the nanoindenter from the peak nanoindentation load. In this unloading stage, the nanogroove fully recovered to its original shape except for the small cutting scars left on the nanogroove edges, which did not contribute much to the residual plastic deformation displacement. According to the three-sided pyramidal ge260
DOI: 10.1021/nl9034187 | Nano Lett. 2010, 10, 255-262
Acknowledgment. This work is supported by the National Science Foundation (CMMI-0653651, CMMI-0824728, and EPS-0296165), the Army Research Office (W911NF-080299), the ACS Petroleum Research Fund (ACS PRF 40450AC10), and the University of South Carolina NanoCenter.
ometry of the Berkovich diamond nanoindenter used in this study, for a CBO nanogroove with an inner diameter of 260 nm, when the indenter was in contact with the CBO nanogroove edges, the corresponding normal height of the indenter embedded in the nanogroove was calculated to be about 15 nm, considering the bluntness of the indenter. The indenter tip did not touch the bottom of the nanogroove in the whole indentation travel with a displacement of 24.6 nm. The load-displacement curve in the second stage represents elastic deformation of the nanogroove and can be used to derive its elastic properties. On the basis of the elastic beambending theory, the relationship between elastic modulus E and the elastic deformation of a CBO nanogroove with hemicirque cross section can be described as44
E)
¯3 3 FR π-2 4Id 4
(
)
Supporting Information Available. Detailed X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) analyses. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES AND NOTES (1) (2) (3) (4) (5)
(3)
(6)
¯ )(1/ where the moment of inertia I ) (π/8)(R4 - r4) and R 2)(R + r) for a hemicirque shaped nanogroove. For a CBO nanogroove with R ) 200 nm and r ) 130 nm, the elastic modulus E was estimated to be 1.0 ( 0.2 GPa. This is also confirmed by Hertzian indentation performed directly on the bottom of the nanogroove using a sharper AFM diamond indenter. Compared with its bulk counterpart, the elastic modulus was decreased by approximately 99%. This is due to the absence of hydrated H2O and formation of stacking faults in the CBO nanogrooves. During the annealing process, the H2O molecules evaporated from the CBOH nanobelts, and then the stacking faults were formed in the CBO nanogrooves to compromise the crystal lattice changes. This resulted in a loose stacking package in the crystal structure and made the CBO nanogrooves much easier to be deformed, thereby leading to the reduction of elastic modulus. Schaefer et al.19 have found that the presence of structural defects (such as stacking faults, dislocations, and twins) largely lowered the elastic modulus of gold nanoclusters. Therefore, by controlling the compositions (with or without hydrated H2O) and microstructure (with or without stacking faults) of the nanomaterials, we should be able to tailor their mechanical properties. In summary, we synthesized single crystalline CBOH nanobelts for the first time using a facile catalyst-free hydrothermal method at low temperature. TEM and SAED characterizations showed that the CBOH nanobelts grew along the [104] direction. After being annealed at 400 °C for 10 h, the CBOH nanobelts transformed into curve-shaped CBO nanogrooves with ordered stacking fault arrays along the [1¯10] direction. Nanoindentation tests performed on individual CBOH nanobelts and CBO nanogrooves revealed that the CBOH nanobelts, with the presence of hydrated H2O, achieved 28.7% increase in elastic modulus, whereas the stacking faulted CBO nanogrooves, with the absence of hydrated H2O, exhibited 99% decrease in elastic modulus, compared to bulk CBO. These findings are significant for tailoring the mechanical properties of nanomaterials by controlling their compositions and microstructure. © 2010 American Chemical Society
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DOI: 10.1021/nl9034187 | Nano Lett. 2010, 10, 255-262
