J. Phys. Chem. C 2009, 113, 18689–18698
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Process Map for the Hydrothermal Synthesis of r-Fe2O3 Nanorods Trevor P. Almeida,† Mike Fay,‡ Yanqiu Zhu,*,† and Paul D. Brown† DiVision of Materials, Mechanics and Structures, Faculty of Engineering and Nottingham Nanotechnology and Nanoscience Centre, UniVersity of Nottingham, UniVersity Park, Nottingham, NG7 2RD, U.K. ReceiVed: July 24, 2009; ReVised Manuscript ReceiVed: September 17, 2009
A “process map” for the hydrothermal synthesis (HS) of single crystalline R-Fe2O3 nanorods from aqueous FeCl3 is presented, as a function of temperature, time, and phosphate concentration, as assessed using the combined techniques of X-ray diffractometry, transmission electron microscopy, selected area electron diffraction, Fourier transform infrared spectrometry, and X-ray photoelectron spectroscopy. The process map provides insight into the nature of intermediate β-FeOOH nanorod precipitation, dissolution and subsequent R-Fe2O3 growth, along with the effect of PO43- anion concentration on the R-Fe2O3 particle shape. Increasing the processing temperature in the absence of a surfactant promoted the dissolution of initially formed β-FeOOH nanorods and the nucleation and growth of equiaxed R-Fe2O3 nanoparticles with rhombohedral morphology. Increasing additions of phosphate surfactant resulted in a shape change of the R-Fe2O3 nanoparticles into lenticular R-Fe2O3 nanorods with increasing aspect ratio but with progressive inhibition of R-Fe2O3 phase formation. Increasing the synthesis temperature in the presence of PO43- anions was associated with the recovery of well-defined single crystal, lenticular nanorods. Increasing the time of synthesis in the presence of PO43- anions was similarly associated with the progressive formation and dissolution of β-FeOOH and the growth of well-defined lenticular R-Fe2O3 nanorods. An HS processing temperature of 200 °C and an Fe3+-PO43- molar ratio of 31.5 yielded optimized crystalline lenticular R-Fe2O3 nanorods with an aspect ratio of ∼7. Chemical analysis indicated that some P was retained within the bulk of the developed R-Fe2O3 nanorods. 1. Introduction One-dimensional (1D) nanostructures have attracted considerable attention due to their novel magnetic properties which are greatly dependent on nanorod size and shape.1 Weakly ferromagnetic R-Fe2O3 (hematite) is of particular interest as a multifunctional, nanostructured material and has been investigated extensively for a variety of applications including photocatalysis,2 gas sensing,3,4 magnetic recording,4 drug delivery,5 tissue repair engineering,6 and magnetic resonance imaging,7 along with lithium-ion batteries,4 spin electronic devices,8 and pigments,9 owing to its environmentally friendly properties, low processing cost, high resistance to corrosion and excellent thermodynamic stability in comparison with other iron oxide phases. Several production methods such as sol-gel processing,10 microemulsion,11 forced hydrolysis,12 hydrothermal synthesis (HS),13 and chemical precipitation14 have been developed for the fabrication of R-Fe2O3 nanoparticles. In particular, HS offers functional control over the size and shape of the particles at relatively low reaction temperatures and short reaction times, providing well-crystallized reaction products with high homogeneity and definite composition.15 Indeed, many R-Fe2O3 nanostructures with varied morphologies have been synthesized successfully using the hydrothermal approach, including nanorods,16 nanotubes,17 nanorings,17,18 hollow nanospheres,19 nanosheets,20 and hollow core/shell hierarchical nanostructures.20 * To whom correspondence should be addressed. E-mail: yanqiu.zhu@ nottingham.ac.uk. † Division of Materials, Mechanics and Structures, Faculty of Engineering. ‡ Nottingham Nanotechnology and Nanoscience Centre.
Aqueous iron(III) chloride (FeCl3) solution is a simple precursor for the formation of R-Fe2O3 nanoparticles, whereby an intermediate phase of β-FeOOH (akaganeite) is produced prior to R-Fe2O3 precipitation.21 The presence of surfactant phosphate anions (PO43-) is found to increase the aspect ratio of the R-Fe2O3 nanoparticles, resulting in single crystal lenticular nanorods.22 In particular, Sugimoto et al.,23,24 using a sol-gel approach and an FeCl3 precursor, have investigated the formation mechanisms of such “spindle-type” R-Fe2O3 nanorods and demonstrated control over their size and shape. Similarly, Hu et al.,25 using a microwave hydrothermal technique, have shown how the FeCl3 and PO43- concentrations can alter the size and shape of R-Fe2O3 spindles, rings, and platelets. The sol-gel method, however, can be very time-consuming with syntheses lasting up to several days, while the microwave method is considered effective yet complicated alongside the HS process that requires simply the heating of a Teflon-lined steel autoclave during the production of R-Fe2O3 nanorods.26-28 Nevertheless, a gap in knowledge remains with regards to the distinct stages of the HS growth of R-Fe2O3 nanoparticles and nanorods, as a function of time, temperature, and surfactant concentration. In this context, a comprehensive HS process map is presented, providing insight into the nature of β-FeOOH precipitation, dissolution, and subsequent R-Fe2O3 growth. In particular, emphasis is given to the effect of PO43- anion concentration on the development of R-Fe2O3 particle shape. 2. Experimental Section For the purpose of the hydrothermal synthesis of nanostructured iron oxide, 0.2 mL of FeCl3 aqueous solution (45% pure FeCl3; Riedel-de Haen, Germany), further diluted in 40 mL of
10.1021/jp907081j CCC: $40.75 2009 American Chemical Society Published on Web 10/05/2009
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J. Phys. Chem. C, Vol. 113, No. 43, 2009
Almeida et al.
TABLE 1: Hydrothermal Synthesis Processing Variables and a Summary of the Reaction Products, Morphologies, Dimensions and Aspect Ratios of the r-Fe2O3 Particles sample NH4H2PO4/ molar ratio temperature/ process no. mg Fe3+-PO43°C time/min S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19
0 0 0 0 0 1 2 3 4 5 3 3 3 3 3 3 3 3 3
94.4 47.2 31.5 23.6 18.9 31.5 31.5 31.5 31.5 31.5 31.5 31.5 31.5 31.5
100 120 140 160 200 160 160 160 160 160 120 140 180 200 220 240 200 200 200
120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 120 30 60 90
reaction product β-FeOOH β-FeOOH β-FeOOH R-Fe2O3 R-Fe2O3 R-Fe2O3′ R-Fe2O3 β-FeOOH β-FeOOH β-FeOOH β-FeOOH β-FeOOH R-Fe2O3 R-Fe2O3 R-Fe2O3 R-Fe2O3 β-FeOOH β-FeOOH R-Fe2O3
morphology
nanorods + R-Fe2O3 nanorods + pseudocubes + R-Fe2O3 nanorods + pseudocubes pseudocubes agglomerated pseudocubes pseudocubes/round/ellipsoidal ellipsoidal nanoparticles + R-Fe2O3 nanorods + partial lenticular nanorods + R-Fe2O3 nanorods + partial lenticular nanorods + R-Fe2O3 nanorods + partial lenticular nanorods nanorods and nanoparticles + R-Fe2O3 nanorods + partial lenticular nanorods partial lenticular nanorods fully formed lenticular nanorods fully formed ellipsoidal nanoparticles fully formed rectangular nanoparticles nanorods and nanoparticles + R-Fe2O3 nanorods + partial lenticular nanorods partial lenticular nanorods
distilled water and mixed with and without an ammonium dihydrogen-phosphate (99.999% NH4H2PO4; Sigma-Aldrich, U.K.) surfactant, was mechanically stirred in a 125 mL Teflonlined steel autoclave. The autoclave was sealed and inserted into a temperature-controlled furnace at the reaction temperature. The controlling parameters of reaction temperature (100-240 °C), time (30-120 min), and surfactant concentration (0-5 mg phosphate addition) were varied in a systematic fashion, as summarized in Table 1. The autoclave, once removed from the furnace, was allowed to cool down to room temperature naturally. The synthesized reaction products were deposited straight from solution onto single crystal silicon substrates for the purpose of structural characterization using a Siemens D500 X-ray diffractometer (Cu KR radiation; θ/2θ diffraction geometry). The crystalline reaction products were identified using DIFFRACplus software. For the purpose of survey transmission electron microscopy (TEM) investigation, the HS product suspensions were deposited straight onto lacey carbon/copper mesh support grids (Agar Scientific Ltd., U.K.) for a generalized impression of the reaction products present. Conventional diffraction contrast imaging was performed using a Jeol 2000fx transmission electron microscope operated at 200 kV. Energy dispersive X-ray (EDX) analysis (ISIS, Oxford Instruments, Abingdon, U.K.) provided information on the elemental constituents, while selected area electron diffraction (SAED) patterns allowed for phase identification and an appraisal of the relationship between nanorod morphology and crystallographic orientation, which was also modeled using Carine software.29 For the purpose of detailed TEM and EDX investigation of individual R-Fe2O3 nanoparticles, the HS product suspensions were centrifuged for 6 min at 6000 rpm, cleaned with acetone, and dispersed using an ultrasonic bath before deposition onto support grids. Digitised SAED negatives were processed using ImageJ software (National Institutes of Health, U.S.A.). Further, X-ray photoelectron spectroscopy (XPS) (VG Scientific ESCALab; Al (anode) X-ray source) was utilized to investigate the surface chemistries of the nanoparticles, while Fourier transform infrared (FT-IR) transmittance spectroscopy (Bruker Tensor, Germany; OPUS Spectroscopy Software) provided information on the covalent bonding within the nanostructures.
dimension (l/w)/nm
