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J. Phys. Chem. C 2009, 113, 9928–9935
Shape-Controlled Synthesis of Single-Crystalline Fe2O3 Hollow Nanocrystals and Their Tunable Optical Properties H. M. Fan,†,* G. J. You,† Y. Li,† Z. Zheng,‡ H. R. Tan,§ Z. X. Shen,‡ S. H. Tang,† and Y. P. Feng†,* Department of Physics, National UniVersity of Singapore, 2 Science DriVe 3, 117542, Singapore, DiVision of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological UniVersity, 637616, Singapore, and Institute of Materials Research and Engineering, 3 Research Link, 117602, Singapore ReceiVed: March 8, 2009; ReVised Manuscript ReceiVed: April 19, 2009
Single-crystalline R-Fe2O3 hollow nanocrystals from nanotube to nanoring have been successfully synthesized by a facile hydrothermal method. Size and shape control of the hollow nanocrystals is achieved by simple adjustments of reactants’ concentration and molar ratio without any surfactant assistance. The steady absorption spectra indicate a size-dependent blue shift of these hollow nanocrystals. The femtosecond optically heterodyne optical Kerr effect measurements show that the sample that has the smallest diameter possesses the largest third-order nonlinear optical susceptibility. This is ascribed to the remarkably enhanced local electric field in the small particle in accord with finite-difference time-domain simulations. These results reveal that both size and shape of these hollow nanocrystals have significant contributions to their optical response. Introduction Iron oxides are the versatile materials existing with three common forms, hematite, maghemite, and magnetite. Among them, hematite (R-Fe2O3) is the most stable structure in nature with an indirect charge transfer bandgap of 1.9-2.0 eV.1 Nanoscale R-Fe2O3 has displayed many important applications in photonics,2 gas sensors,3 Li-ion batteries,4 and water treatment.5 Because of their widely used applications, the syntheses of various R-Fe2O3 nanocrystals including spherical nanocrystals,6 nanorods/nanowire,1,7 and nanoflakes8 have been reported. Because of their intriguing properties and unique applications, hollow structured nanocrystals such as nanotubes and nanorings, which are a special type of nanostructures, have been the subject of intensive research.9-12 Though considerable effort has been made in fabrication of hollow structured inorganic nanocrystals in the past few years, 13-15 the successive modulation of size or shape of hollow structured R-Fe2O3 nanocrystals has yet to become reality. Tailoring size or shape in such a system not only represents the transformation from a 1D nanotube to a 0D nanoring but also represents the dramatic change in their properties due to the spatial geometry effect. For instance, when the shape transforms from a nanoring to a high aspect nanotube in a nanomagnet system, the longitudinal magnetic field turns out to be a twisted bamboo state.16 Therefore, it is necessary to fabricate the different-shaped hollow R-Fe2O3 and investigate their shape-dependent physical properties. Semiconductor nanocrystals have shown unique optical properties that could be tuned by size and shape. For example, multicolor emissions in CdSe nanocrsytals system have been achieved by tailoring their size.17 Substantive investigations on the optical properties in varieties of semiconductor nanocrystals have been reported;18-20 most of them, however, focused on * To whom correspondence should be addressed. E-mail: phyfhm@ nus.edu.sg (H.M.F.),
[email protected] (Y.P.F.). † National University of Singapore. ‡ Nanyang Technological University. § Institute of Materials Research and Engineering.
the size-dependent optical properties where the size of investigated nanocrystal is usually comparable to their exciton Bohr radius (strong confinement region). The shape effect on the optical response of bigger nanocrystals (weak confinement region) is limited. Unlike the II-VI group semiconductor, hematite is a representative transition-metal oxide with strong electron-electron correlation and electron-phonon coupling, which lead to complex electronic structures and rich optical properties.21 In particular, R-Fe2O3 has nearly the largest refraction index and the highest third-order nonlinear optical susceptibility among inorganic materials.22 Hence, investigations on the third-order nonlinear properties of different-shaped hollow Fe2O3 nanocrystals to understand the underlying mechanism of how shape affects their optical properties are of great interest in both fundamental research and technological applications. Methods used to determine third-order nonlinear optical susceptibility (χ(3)) of the nanocrystal systems are diverse, such as Z-scan, 23,24 optical Kerr effect (OKE), 25 third-harmonic generation (THG),22 four wave mixing (FWM),2 and so forth. OKE and FWM methods can only measure the modulus of χ(3). Although the single-beam Z-scan method can provide both the real and the imaginary parts of χ(3), it cannot give dynamic information, which is important for designing the optical switching devices. The optically heterodyne optical Kerr effect (OHD-OKE), which is through the introduction of a local oscillator field with or without 90° optical phase biasing (optically heterodyne) in a homodyne OKE system, offers the possibility to separately investigate the sign, magnitude, and response of both real and imaginary parts of χ(3). However, the femtosecond OHD-OKE measurements on Fe2O3 nanocrystals have never been reported so far. In the present work, the single-crystalline R-Fe2O3 hollow nanocrystals from nanotube to nanoring were synthesized by a simple hydrothermal process without any surfactants. The size and shape control was achieved by a simple adjustment of the reactants’ concentration and molar ratio. The steady absorption
10.1021/jp9020883 CCC: $40.75 2009 American Chemical Society Published on Web 05/07/2009
Single-Crystalline Fe2O3 Hollow Nanocrystals and femtosecond OHD-OKE measurements were carried out to study the size- and shape-dependent optical response of these hollow nanocrystals. Experimental Section The R-Fe2O3 nanotubes were synthesized by a hydrothermal treatment of FeCl3 solution in the presence of NH4H2PO4. In a typical experimental procedure for 260 nm R-Fe2O3 nanotubes, 1.60 mL of aqueous FeCl3 solution (0.5 M) and 1.44 mL of aqueous NH4H2PO4 solution (0.02 M) were mixed with vigorous stirring. Distilled water was then added to a final volume of 40 mL. After stirring for 10 minutes, the mixture was transferred into a Teflon-lined stainless-steel autoclave with a capacity of 50 mL for hydrothermal treatment at 220 °C for 60 h. The autoclave then cooled down to room temperature naturally, the red precipitate was separated by centrifugation, washed with distilled water and absolute ethanol, and dried under vacuum at 80 °C. The obtained sample was labeled S1. By simple adjustment of the reactant concentration, the different-sized R-Fe2O3 hollow nanocrystals are prepared. For samples S2, 0.8 mL of aqueous FeCl3 solution (0.5M) and 0.72 mL of aqueous NH4H2PO4 solution (0.02M) were used. Also, 0.4 mL of aqueous FeCl3 solution (0.5M) and 1.42 mL of aqueous NH4H2PO4 solution (0.02M) were used for the preparation of samples S3. All of the reagents were of analytical purity and purchased from Sigma-Aldrich Chemical, Co. The morphology and structure of the samples were determined using a field-emission scanning electron microscope (FESEM, JEOL JSM-6700F) and a field-emission transmission electron microscope (TEM, JEOL, JEM 2010, accelerating voltage 200 KV). The crystal structure of the hollow nanocrystals was examined by a Bruker D/MAX 2500 X-ray diffractometer with ˚ ). Raman scattering was carried Cu K radiation (λ ) 1.54056 A out by a micro-Raman system (Jobin-Yvon T64000). The laser power was approximately 0.2 mW. For the optical measurements, the samples (5 mg/mL aqueous solution) were spin-coating on the 2 × 2 cm2 quartz substrates to form the thin film with the average thickness of ∼0.5 µm. Steady absorption spectra were measured by a UV-vis spectrophotometer (Shimadzu, UV-1700). The two-color OHD-OKE experimental arrangement is similar to that of a standard pump-probe experiment and the detail setup has been described in refs 25 and 26. The ultrashort 800 nm laser pulses with a pulse duration of 60 fs, pulse energy of 1 mJ, and a repetition rate of 1 kHz were delivered from a Ti:sapphire regenerative amplifier system (Coherent, Legend). The laser beam was split into two portions at a 9:1 ratio. The stronger beam was frequency-doubled in a BBO crystal to generate 400 nm second harmonic generation (SHG) pulses used as the pump beam and the weaker beam was used as the probe beam. To separate real and imaginary parts of the third-order nonlinear optical susceptibility χ(3), a quarter-wave plate (QP) with its optical axis parallel to the polarization of probe beam can optionally be inserted after the sample. By rotating the polarizer with a small angle (defined as the heterodyning angle Φ), a local oscillator filed was introduced into the Kerr signal. The real or imaginary component of heterodyned Kerr signal can be selectively enhanced by the local oscillator filed with or without presence of the QP. The basic principle of OHD-OKE can be described simply as
J. Phys. Chem. C, Vol. 113, No. 22, 2009 9929
I ∝ (χ(3))r(im) × Φ
(1)
2 |χ(3) | ) √(χ(3))2r + (χ(3))im
(2)
where I is the intensity of heterodyned Kerr signal at zero delay time and (χ(3))r(im) denotes the real (subscript r) and imaginary (subscript im) part of χ(3). From formula (1), the intensity of heterodyned Kerr signal at different Φ should be on a line in the coordinate plane, and value of (χ(3))r(im) can be deduced from the slope of the line that is denoted as Z2 in formula (3). Taking a stand material as the reference, the value of (χ(3))r(im) of the sample could be calculated by the use of formula (3)
( )( )() s z2,r(im)
s ref (χ(3))r(im) ) (χ(3))r(im) ×
R)
ref z2,r
×
Lref nref × Ls ns
e-3RL / 2[1 - e-RL] RL
-1 / 2
×
1 R2
(3) (4)
Here, superscripts s and ref denote sample and reference, respectively, and n represents refractive index. L is the overlapped length of pump and probe beams in sample or reference, whereas R is the linear absorption coefficient of the sample. In the experiment, a quartz plate with thickness of 1 mm was chosen as the reference. Results and Discussion Figure 1 shows the SEM images of the as-prepared hematite hollow nanocrystals. As can be seen in Figure 1, the large-scale hollow structured R-Fe2O3 nanocrystals with uniform size have been fabricated by this facile hydrothermal method and the morphology evolved from nanotubes (S1) to nanorings (S2 and S3). The average length of these samples varies from 260 nm (S1) and 50 nm (S2) to 10 nm (S3). The average outer diameters are 98 nm (S1), 74 nm (S2), and 150 nm (S3), and the thicknesses are 16 nm (S1), 14 nm (S2), and 33 nm (S3) as shown in the insets of Figure 1. Figure 2 shows the TEM and HRTEM images of these R-Fe2O3 hollow nanocrystals. The HRTEM images and the selective-area electron diffraction (SAED) patterns in parts a-c of Figure 2 indicate their singlecrystal nature. The lattice spacing about 4.54 and 2.5 Å for (0003) and (12- 10) planes of the trigonal Fe2O3 can be readily resolved. The TEM analyses also reveal that the axis of all hollow nanocrystals (ring/tube axis) is in the [0001] direction (c axis). The phase purity and crystal structure of the R-Fe2O3 hollow nanocrystals have been examined by XRD and Raman spectroscopy. As shown in part a of Figure 3, all of the diffraction peaks can be exclusively indexed as the trigonal R-Fe2O3 (JCPDS 87-1165), and no other impurities are observed. The Raman spectra of the samples are shown in part b of Figure 3, with three peaks present in the range of 150-800 cm-1. As 6 crystal space group with two A1g hematite belongs to the D3d 27,28 modes and five Eg modes; the peaks at 225 cm-1 are assigned to the A1g mode and the peaks at 290 and 407 cm-1 are assigned to Eg modes. Despite the fact that the red shift of Raman peaks in nanoscale transition-metal oxides were observed due to quantum size effect,18 it is not found in our case because the wall thickness of samples are quite big compared with these nanocrystals (size
