Nanoribbons and Their Moiré Patterns - American Chemical Society

Article pubs.acs.org/JPCC

Ultrathin α-Fe2O3 Nanoribbons and Their Moiré Patterns Rui Xu,† Hui Yan,† Wenyu He, Ying Su, Jia-Cai Nie, and Lin He* Department of Physics, Beijing Normal University, 100875 Beijing, People's Republic of China ABSTRACT: Ultrathin α-Fe2O3 nanoribbons several tens of nanometers in thickness were synthesized by heating iron foils under atmospheric conditions. Our result indicates that the growth rate of the crystal planes of these α-Fe 2 O 3 nanostructures follows {110} > {104} ≫ {2̅10}. Moiré patterns with different periodicities were observed due to the misorientations between the nanoribbons. Because of the finite size effect, the Morin temperature of the α-Fe2O3 nanoribbons with a thickness of 10 nm is reduced from 264 K of the bulk phase to only 42 K.

■

INTRODUCTION α-Fe2O3, the most stable iron oxide under ambient conditions, exhibits a rhombohedral structure and an indirect band gap of 2.1 eV.1 It is an environmental friendly material that has been used widely due to its catalytic, sensing properties, fieldemission application, and photocatalyst.2−5 Besides that, magnetic properties of the α-Fe2O3 crystal also have attracted much attention. 6,7 The bulk hematite α-Fe 2 O 3 is an antiferromagnet with a Neel temperature (TN) ∼ 960 K. It undergoes a spin-reorientation transition, known as the “Morin transition”, at the Morin temperature, TM ∼ 264 K. Above TM, the moments lie in the basal plane (111) with a slight canting angle resulting in a weakly net magnetic moment in the plane. Below TM, the two magnetic sublattices are reoriented along the rhombohedral [111] axis, showing exactly the antiparallel configuration as expected for an antiferromagnetic ordering. Recently, many strategies, such as hydrothermal,8−11 thermal oxidation method,12−15 and the template-based method,16−18 have been developed to synthesize the α-Fe2O3 nanostructure because of its unique properties and potential wide-ranging applications. In this work, we report the thermal oxidation method to produce ultrathin α-Fe2O3 nanoribbons. The structure, morphologies, growth mechanisms, and magnetic properties of the ultrathin α-Fe2O3 nanoribbons were studied systematically. According to our experimental results, the growth rate of the crystal planes of these α-Fe2O3 nanostructures follows {110} > {104} ≫ {2̅10}, and the impurities of iron substrates play a dominant role in the growth of the nanoribbons. Moiré patterns with different periodicities were observed by transmission electron microscopy (TEM) because of misorientation between the ultrathin nanoribbons. Because of the finite size effect, the Morin temperature of the α-Fe2O3 nanoribbons with a thickness of 10 nm is reduced from 264 K of the bulk phase to only 42 K.

RESULTS AND DISCUSSION Figure 1a−d shows four typical SEM images of the α-Fe2O3 nanostructures prepared by heating iron foils for 6 h at 280, 300, 400, and 500 °C, respectively. Both the width and the thickness of the nanostructures increase with increasing the heating temperature. For the samples synthesized below 400 °C, it is observed that ultrathin nanoribbons with a large surface coverage have grown on the substrates. By increasing the growth temperature to 500 °C, only α-Fe2O3 nanoflakes can be obtained, as shown in Figure 1d. The heating duration also plays a similar role as the temperature in controlling the morphologies of the obtained samples (see Figure 2). Hence, the shape of the nanostructures can be controlled by varying the growth temperature and duration. According to the TEM studies, which will be elaborated subsequently, the growth rate of the crystal planes of these α-Fe2O3 nanostructures follows {110} > {104} ≫ {2̅10}. The width and thickness of the α-Fe2O3 nanoribbons were further characterized by AFM measurements. Figure 3a shows a typical AFM image of two α-Fe2O3 nanoribbons synthesized at 400 °C. The profile line of the two nanoribbons reveals that the thickness is about 30 nm. By combining use of the SEM and AFM images, we obtained the average thickness and width of αFe2O3 nanoribbons synthesized at different temperatures. Figure 3c summarizes the corresponding width and thickness

EXPERIMENTAL METHODS The ultrathin α-Fe2O3 nanoribbons were synthesized by heating carbon-doped iron foils (the purity of the iron in the

Received: January 17, 2012 Revised: March 1, 2012 Published: March 7, 2012

foils is about 98.7%) in a tube furnace under atmospheric conditions. Similar to the method reported in previous papers,19,20 we obtained α-Fe2O3 nanoribbons and nanoflakes with exposing (21̅ 0) surface planes by controlling the heating temperature. The heating duration is the other key factor that determines the morphologies of the obtained sample. The morphologies of the samples were studied by scanning electron microscopy (SEM), atomic force microscopy (AFM), TEM, and high-resolution TEM (HRTEM).

■

■

© 2012 American Chemical Society

6879

dx.doi.org/10.1021/jp300594m | J. Phys. Chem. C 2012, 116, 6879−6883

The Journal of Physical Chemistry C

Article

Figure 1. SEM images of four typical samples synthesized by heating the iron foils at (a) 280, (b) 300, (c) 400, and (d) 500 °C for 6 h. Nanoribbons can be observed on the surface of iron foil heated below 400 °C. Above 400 °C, only nanoflakes can be obtained. The scale bar is 400 nm.

Figure 2. SEM images of four typical samples synthesized by heating the iron foils at 400 °C for (a) 5 min, (b) 30 min, (c) 1 h, and (d) 6 h. The scale bar is 200 nm.

as a function of temperature. Both the width and the thickness increase quickly with an increase in the temperature. The obtained nanostructures change from nanoribbons at low temperature to nanoflakes or nanowalls at high temperature. To study the growth mechanism of the α-Fe2O3 nanostructures, we carried out the following controlled experiments. Figure 4 shows two typical SEM images of ultrathin α-Fe2O3 nanoribbons grown on two iron foils with different impurities. The purities of the iron foils are 99.9 and 98.7% for the samples shown in Figures 4a,b, respectively. The main impurity is carbon according to energy dispersive spectrometer (EDS) measurements (not shown). Both of the two samples were synthesized at 300 °C for 6 h. Obviously, we obtained quite

different surface coverage densities of the nanoribbons. It suggests that the carbon in the iron foil is vital for the growth of the α-Fe2O3 nanostructures.21,22 Rao et al.14 recently have suggested a solid-phase oxidation as the possible mechanism for the growth of α-Fe2O3 nanostructures. Their experimental results suggest that carbon dioxide and water vapor may affect the diffusive rate of iron and consequently impact the growth of α-Fe2O3 nanostructures. In our experiment, the carbon in the substrates could be oxidated to form carbon dioxide during the heating process. The result shown in Figure 4, to some extent, gives direct experimental evidence, for the carbon dioxide promotes the rate of iron diffusion and influences the growth rate of α-Fe2O3 nanostructures. 6880

dx.doi.org/10.1021/jp300594m | J. Phys. Chem. C 2012, 116, 6879−6883

The Journal of Physical Chemistry C

Article

Figure 3. (a) Typical AFM image of two nanoribbons synthesized at 400 °C. (b) The profile line of the two nanoribbons in panel a. (c) Thickness (the left Y-axis) and width (the right Y-axis) of the α-Fe2O3 nanoribbon as a function of synthesized temperatures.

Figure 4. SEM images of nanoribbons grown on two iron foils with different purities. Both of the samples were heated at 300 °C for 6 h. (a) The purities of the iron foils are (a) 99.9 and (b) 98.7%. The scale bar is 1 μm.

The ultrathin α-Fe2O3 nanostructures were further characterized by TEM measurements. Figure 5 shows typical TEM and HRTEM images of an α-Fe2O3 nanoribbon synthesized at 280 °C. The nanoribbon is single crystal. The fringe spacing ∼0.25 nm along the growth direction of the nanoribbon concurs well with the interplanar spacing of the (110) plane; that is, the α-Fe2O3 nanoribbons grow along the [110] direction. This is consistent with that reported in previous study.12 The direction of the (104) plane with fringe spacing ∼0.27 nm is about 57.3° relative to that of the (110) plane, as shown in Figure 5c. Usually, the average length is much larger than the width of the nanoribbon, which indicates that the growth rate of the crystal plane {110} is much faster than that of the {104} plane. The α-Fe2O3 nanostructures synthesized below 400 °C are almost transparent in the SEM measurements, which indicate that the nanoribbons are ultrathin. This is further confirmed in our TEM measurements. It is easy to observe moiré patterns during the TEM measurements when two ultrathin nanoribbons overlap with a twist angle between the two [110] directions. Figure 6a,d shows two typical low magnification TEM images with moiré patterns. Figure 6b,e shows the corresponding enlarged views in the framed regions of Figure 6a,d. It can be seen that the misorientations between the top and the sublayer nanoribbons result in the moiré patterns. The periodicity D of the moiré patterns is related to the lattice constant of α-Fe2O3 nanoribbon d and the twist angle θ by the Rayleigh relation D = d/[2 sin(θ/2)].23,24 The lattice constants of α-Fe2O3 in x- and y-directions are labeled as dx = 0.288 nm and dy = 0.336 nm, respectively. By taking into account the

twist angle 6.5°, as shown in the schematic structural model in Figure 6c, the periodicities D of the moiré patterns are estimated as Dx = 2.54 nm and Dy = 2.96 nm, which are consistent with our experimental results that Dx = 2.6 nm and Dy = 2.8 nm. The Dx and Dy in Figure 6e are measured as 4.12 and 4.42 nm, respectively. Figure 6f shows the corresponding schematic model with a twist angle of 4.3°. The calculated Dx and Dy are 3.83 and 4.47 nm, which are almost consistent with our experimental results. With taking into account that the angle ∼60° between the periodic protuberances of the moiré patterns, the other important information that the α-Fe2O3 nanoribbons expose surface (2̅10) planes can be obtained. According to our experimental result, the thickness is much smaller than the width and the length of the nanoribbon. Therefore, we can conclude that the growth rate of the crystal planes of these α-Fe2O3 nanostructures follows {110} > {104} ≫ {2̅10}. The magnetic properties of the ultrathin α-Fe2O3 nanoribbons were studied using a Quantum Design superconducting quantum interference device. The ultrathin α-Fe2O3 nanoribbons were carefully peeled off from the substrate (the surface of the substrate is oxidated to be Fe3O4). Figure 7a shows the temperature dependence of magnetization, M(T) curves, under zero-field-cooling and field-cooling (ZFC and FC) modes measured from 5 to 300 K. The abrupt change of magnetization at 125 K is attributed to the Verwey transition temperature TV of Fe3O4.25,26 It indicates that there is a residual Fe3O4 in the sample. Figure 7b shows the hysteresis loop measured at 5 K. The saturation magnetization is determined as 6.2 emu/g. By taking into account the saturation magnetization of bulk Fe3O4, 6881

dx.doi.org/10.1021/jp300594m | J. Phys. Chem. C 2012, 116, 6879−6883

The Journal of Physical Chemistry C

Article

Figure 5. (a) Typical TEM image of a α-Fe2O3 nanoribbon synthesized at 280 °C. The width at the bottom and top of the nanoribbon is about 80 and 40 nm, respectively. (b) A typical HRTEM image measured at the boundary of the same α-Fe2O3 nanoribbon. The nanoribbon grows along the [110] direction. (c) A HRTEM image of the inner part of the α-Fe2O3 nanoribbon.

the quantity of residual Fe3O4 is estimated as 6% of the measured sample. The slight residual Fe3O4 most possibly located at the bottom of some α-Fe2O3 nanoribbons.

Figure 7. (a) M(T) curves under ZFC and FC modes. The applied magnetic field is 90 Oe. TV is the Verwey transition temperature of Fe3O4, and TM is the Morin temperature of the ultrathin α-Fe2O3 nanoribbons. (b) M(H) curve at 5 K. The inset shows the M(H) curve in the low-field region.

Remarkably, we observed another abrupt change of magnetization of the M(T) curves at 42 K, which is attributed to the Morin temperature TM of the ultrathin α-Fe2O3 nanoribbons. Because of the finite size effect, the transition temperature of magnetic films,27 nanoparticle,28 and nanowires29 decreases with a decrease in the size of the sample. Usually, the transition temperature is mainly dominated by the smallest confined dimension of the sample. The Morin temperature of α-Fe2O3 nanostructure also decreases with a decrease in the size.30 The relation between the Morin temperature and the smallest confined dimension d can be empirically expressed as

Figure 6. Moiré patterns with different periodicities due to the misorientations between ultrathin α-Fe2O3 nanoribbons. (a and d) Low magnification TEM images with moiré patterns. (b and e): The zoom-in images of the framed regions in panels a and d, respectively. (c and f) Schematic structural models of two misoriented α-Fe2O3 (2̅10) planes with a twist angle. The structure appears more open in this top view when atoms from the two layers are nearly on top of each other. In plane c, the periodicities of the moiré pattern are about 2.6 nm in the x-direction and about 2.8 nm in the y-direction, which corresponds to a twist angle 6.5° between the [110] directions of two nanoribbons. In panel f, the size of moiré pattern is about 4.2 nm, which corresponds to a twist angle of 4.3°.

TM = 264 − 2194/d

(1) 30

where d is expressed in nm and TM in K. Although eq 1 was obtained experimentally in particles, we expect that the relation by eq 1 applies equally well for any α-Fe2O3 system with a reduced dimensionality. The average thickness d of the 6882

dx.doi.org/10.1021/jp300594m | J. Phys. Chem. C 2012, 116, 6879−6883

The Journal of Physical Chemistry C

Article

nanoribbon is estimated as ∼10 nm, which agrees quite well with the result of our SEM and TEM characterizations.

(18) Bachmann, J.; Jing, J.; Knez, M.; Barth, S.; Shen, H.; Mathur, S.; Gosele, U.; Nielsch, K. J. Am. Chem. Soc. 2007, 129, 9554−9555. (19) Sun, Y.; Lv, P.; Yang, J.-Y.; He, L.; Nie, J.-C.; Liu, X.; Li, Y. D. Chem. Commun. 2011, 47, 11279−11281. (20) Yan, H.; Sun, Y.; He, L.; Nie, J.-C. Mater. Res. Bull. 2011, 46, 2120−2124. (21) Wang, F.; Tang, R.; Kao, J. L.-F.; Dingman, S. D.; Buhro, W. E. J. Am. Chem. Soc. 2009, 131, 4983−4994. (22) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59−61. (23) Luchnikov, V.; Kondyurin, A.; Formanek, P.; Lichte, H.; Stamm, M. Nano Lett. 2007, 7, 3628−3632. (24) Warner, J. H.; Rummeli, M. H.; Gemming, T.; Buchner, B.; Briggs, G. A. D. Nano Lett. 2009, 9, 102−106. (25) Verwey, E. J. W. Nature 1939, 144, 327−328. (26) Wright, J. P.; Attfield, J. P.; Radaelli, P. G. Phys. Rev. Lett. 2001, 87, 266401(4). (27) Qiu, Z. Q.; Pearson, J.; Barder, S. D. Phys. Rev. Lett. 1993, 70, 1006(4). (28) He, L.; Chen, C. P.; Wang, N.; Zhou, W.; Guo, L. J. Appl. Phys. 2007, 102, 103911(4). (29) Sun, L.; Searson, P. C.; Chien, C. L. Phys. Rev. B 2000, 61, R6463−R6466. (30) Amin, N.; Arajs, S. Phys. Rev. B 1987, 35, 4810−4811.

■

CONCLUSIONS In summary, we synthesized ultrathin α-Fe2O3 nanoribbons by a thermal oxidation method. The structure, morphologies, growth mechanisms, and magnetic properties of the ultrathin αFe2O3 nanoribbons were studied systematically. The growth rate of the crystal planes of these α-Fe2O3 nanostructures follows {110} > {104} ≫ {2̅10}. Moiré patterns with different periodicities were observed because of the misorientations between the ultrathin nanoribbons. The Morin temperature of the ultrathin α-Fe2O3 nanoribbons with a thickness of 10 nm is reduced from 264 K of the bulk phase to only 42 K due to the finite size effect.

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +86-10-58807900. Fax: +86-10-58800141. E-mail: [email protected]. Author Contributions †

These authors contributed equally to this paper.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Dr. Xuewen Fu and Prof. Zhimin Liao of Peking University for helpful discussions. This work was supported by the National Natural Science Foundation of China (Grant Nos. 10974019, 11004010, 51172029, and 91121012) and the Fundamental Research Funds for the Central Universities.

■

REFERENCES

(1) Wang, R.; Chen, Y.; Fu, Y.; Zhang, H.; Kisielowski, C. J. Phys. Chem. B 2005, 109, 12245−12249. (2) Shekhah, O.; Ranke, W.; Schule, A.; Kolios, G.; Schlogl, R. Angew. Chem., Int. Ed. 2003, 42, 5760−5763. (3) Huo, L.; Li, W.; Lu, L.; Cui, H.; Xi, S.; Wang, J.; Zhao, B.; Shen, Y.; Lu, Z. Chem. Mater. 2000, 12, 790−794. (4) Leland, J. K.; Bard, A. J. J. Phys. Chem. 1987, 91, 5076−5083. (5) Hsu, L.-C.; Yu, H.-C.; Chang, T.-H.; Li, Y.-Y. Appl. Mater. Interfaces 2011, 3, 3084−3090. (6) Catti, M.; Valerio, G.; Dovesi, R. Phys. Rev. B 1995, 51, 7441− 7450. (7) Hansen, M. F.; Koch, C. B.; Mørup, S. Phys. Rev. B 2000, 62, 1124−1135. (8) Wang, X.; Chen, X.; Gao, L.; Zheng, H.; Ji, M.; Tang, C.; Shen, T.; Zhang, Z. J. Mater. Chem. 2004, 14, 905−907. (9) Jia, C.-J.; Sun, L.-D.; Yan, Z.-G.; You, L.-P.; Luo, F.; Han, X.-D.; Pang, Y.-C.; Zhang, Z.; Yan, C.-H. Angew. Chem., Int. Ed. 2005, 44, 4328−4333. (10) Wang, Y.; Cao, J.; Wang, S.; Guo, X.; Zhang, J.; Xia, H.; Zhang, S.; Wu, S. J. Phys. Chem. C 2008, 112, 17804−17808. (11) Ma, J.; Lian, J.; Duan, X.; Liu, X.; Zheng, W. J. Phys. Chem. C 2010, 114, 10671−10676. (12) Fu, Y. Y.; Wang, R. M.; Xu, J.; Chen, J.; Yan, Y.; Narlikar, A. V.; Zhang, H. Chem. Phys. Lett. 2003, 379, 373−379. (13) Wen, X.; Wang, S.; Ding, Y.; Wang, Z. L.; Yang, S. J. Phys. Chem. B 2005, 109, 215−220. (14) Rao, P. M.; Zheng, X. Nano Lett. 2009, 9, 3001−3006. (15) Liao, L.; Zheng, Z.; Yan, B.; Zhang, J. X.; Gong, H.; Li, J. C.; Liu, C.; Shen, Z. X.; Yu, T. J. Phys. Chem. C 2008, 112, 10784−10788. (16) Chen, J.; Xu, L.; Li, W.; Gou, X. Adv. Mater. 2005, 17, 582−586. (17) Suber, L.; Imperatori, P.; Ausanio, G.; Fabbri, F.; Hofmeister, H. J. Phys. Chem. B 2005, 109, 7103−7109. 6883

dx.doi.org/10.1021/jp300594m | J. Phys. Chem. C 2012, 116, 6879−6883