Cite this: CrystEngComm, 2016, 18,
Received 12th March 2016,Accepted 9th May 2016
Structural- and optical-properties analysis ofsingle crystalline hematite (-Fe2O3) nanocubesprepared by one-pot hydrothermal approach
Nur Maisarah Abdul Rashid,a Choonyian Haw,a Weesiong Chiu,*a
Noor Hamizah Khanis,a Aliff Rohaizad,a PoiSim Khiewb and Saadah Abdul Rahman*a
High quality single crystal hematite (-Fe2O3) nanocubes with average dimensions of 40 nm were success-
fully synthesized by a facile one-pot hydrothermal method. Systematic analyses were performed to investi-
gate the morphological-, structural- and optical-properties of the as-synthesized -Fe2O3 nanocubes.
Continuous formation and hourly monitoring towards proper arrangement of single crystal -Fe2O3 nano-
cubes was observed throughout the hydrothermal heating process of 180 C from 4 h to 12 h. The proba-
ble growth mechanism on the formation of cubic nanostructures is also proposed. Electron micrographs
show the cubic -Fe2O3 synthesized at the most optimum 8 h hydrothermal heating duration are indeed
produced in high-yield with a well-defined cubical shape. The typical rhombohedral structure of cubic
-Fe2O3 was evident from the XRD pattern. The SAED pattern indicates that the -Fe2O3 nanocubes are
single-crystalline in nature, with lattice-fringes and a d-spacing value of 3.6 . The optical characterization
reveals that -Fe2O3 nanocubes show strong visible-light absorption with a band gap energy of 2.1 eVwhile the photoluminescence emission spectra depicts a mono-peak centered at 590 nm. Both the SAEDpattern and UV-vis spectra show a strong correlation with the standard -Fe2O3. The as-synthesized
-Fe2O3 single crystal is of high quality that potentially could be used as a visible-light active nanomaterial
in renewable energy device applications.
Rational design and synthesis of metal oxide nanocrystalswith tunable shape and properties have attracted enormousresearch interest for their unique size and shape-dependentintrinsic physicochemical properties. In particular, metal ox-ide semiconductor nanocrystals have been identified as im-portant materials with potential applications in a wide rangeof fields (optical, electrical, magnetic, catalytic, chemical,etc.).15 Hematite (-Fe2O3) is among one of the most interest-ing n-type metal oxide semiconductors with an opticalbandgap of 2.1 eV.6,7 Due to its prevalence, hardness,chemical- and thermal-stability, and environmentally benig-nity, it has become an attractive material in a spectrum ofsemiconducting applications. In addition, its ability to absorblight makes it a particularly attractive material for use in solarenergy conversion.8,9 -Fe2O3 is isostructural with corundum,
Al2O3, which is trigonal (hexagonal scalenohedral, symbol 32/m) with space group R3c and lattice parameters a = 5.0356 ,c = 13.7489 , having Fe3+ ions occupy 2/3 of its octahedralsites that are confined by the nearly ideal hexagonal closed-pack O lattice.10 The dense hexagonal close packing of oxygencombined with the interstitially positioned iron yields a verydense structure (5.26 g cm3) exhibiting a high polarizabilityand high refractive index (3.15).11 Attributed to its layeredstructure also generates complex behaviour when interactingwith photons and electrons, it is of interest among re-searchers to exploit -Fe2O3 nanocrystals in numerous prom-ising applications, such as gas sensors,12 catalysts/photo-catalysts,13 electrode materials in lithium secondarybatteries,14 magnetic recording media,15 photo-assistedelectrolysis of water,16 and optical and electromagnetic de-vices as well as environmental remediation in wastewatertreatment.1719
It is noted that the morphology and size of -Fe2O3 have agreat impact on its intrinsic physicochemical properties andthus determine its application.20 Therefore, much effort hasbeen made in the design of -Fe2O3 materials with a desiredstructure and morphology such as zero-dimensional (0D)nanoparticles,21 one-dimensional (1D) nanowires,22 nano-belts,23 nanorods,24 nanotubes,25 two-dimensional (2D)
4720 | CrystEngComm, 2016, 18, 47204732 This journal is The Royal Society of Chemistry 2016
a Low Dimensional Materials Research Centre, Department of Physics, University
of Malaya, 50603 Kuala Lumpur, Malaysia. E-mail: email@example.com,
firstname.lastname@example.orgDepartment of Chemical Engineering, Faculty of Engineering, University of
Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor Darul
CrystEngComm, 2016, 18, 47204732 | 4721This journal is The Royal Society of Chemistry 2016
nanorings,26 nanoflakes,27 three-dimensional (3D) nano-cubes,28 urchin-like nanostructures,29 nanoflowers,30 and etc.In particular, well-defined single crystalline -Fe2O3 nano-cubes exposing specific facets are demonstrated to be ex-tremely notable due to the highly reactive surfaces and excel-lent electron mobility.3133 In terms of crystallinity nature, asingle crystalline -Fe2O3 has a continuous crystal lattice forits entire nanostructure. The crystal lattice arrangement isendless until the edge of the atomic arrays, with no grainboundaries observed. Such a continuous atomic array indeedrenders several advantages especially the aspect of its congru-ent electronic properties. For instance, the absence of grainboundaries in the crystal structure are found to be able to ac-celerate the movement of electric charges in a circuit espe-cially in solar cells and PEC cells.34 Additionally, single crys-tallinity also allows a longer lifetime of electronhole pairswhich could enhance the efficiency in the aforementionedapplications.35 As in the case of gas sensor application, a sin-gle crystalline -Fe2O3 nanostructure is found to be able to re-duce instability problems associated with grain coalescenceand drift in electrical properties,36 which alternatively im-prove its sensitivity and stability. Therefore, it is worth notingthat the advantageous characteristics can be harnessedthrough such a structure that would enhance and diversifyits performance.
To date, a myriad of chemical and physical methods havebeen developed for the synthesis of single crystalline -Fe2O3nanocubes. These methods include a sonochemical route,37
thermal decomposition of inorganic precursors,38 forced hy-drolysis,39 electrospinning,40 and a hydrothermal approach.41
Compared with other fabrication techniques, hydrothermalsynthesis is considered as the most robust method and hasattracted a rapidly increasing interest for the preparation ofsingle crystalline -Fe2O3 nanocubes due to several merits:the products have a high purity and excellent crystallinity,morphologies can be easily tuned, a relatively low reactiontemperature and readily available for scale-up production.For example, Pu and co-workers have successfully synthesized-Fe2O3 nanocubes by a templating method, in which theyemployed the cationic capping agent/surfactant cetyltrimethylammonium bromide (CTAB) and showed that con-centration variation of the precursor could lead to differentsizes and shapes of -Fe2O3. In this study, the rod-like-FeOOH precursors were firstly obtained by varying FeCl3concentration ranging from 0.01 to 0.05 M at 120 C, andthen the precursors aggregated into raft-like particles andthereby transformed to -Fe2O3 cubic particles.
5 Mitra et al.have reported the controlled synthesis of -Fe2O3 microcubicparticles by pH-controlled hydrolysis of Fe(NO3)39H2O withthe use of ethylenediamine as the basic ligand to facilitatethe formation of cubic samples,42 whereas Su et al. used am-monium acetate in FeCl36H2O aqueous solution as the cap-ping agent to produce -Fe2O3 at 160 C for 24 h.
43 In addi-tion to this, Ma and colleagues synthesized single-crystal-Fe2O3 nanocubes by the use of diethylene glycol (DEG) at200 C for 10 h in Li ion battery applications.44 Very recently,
Patra and his group used a proportional amount of sodiumsalicylate and NaOH/water to prepare single crystalline-Fe2O3 nanocubic particles via a hydrothermal route for 36h at 423 K and investigated the facet-dependent photoredoxcatalytic activity.45 However, most of the aforementioned syn-thetic routes tend to involve the use of organic surfactants,which means a much more complicated process includingcomplete template removal at elevated temperature isneeded. Furthermore, solvents used are either potentiallyhazardous or a rather complicated protocol is involved, andsome synthesis methods are often time-consuming (>24 h).It is therefore necessary to investigate further the growth ofsingle crystalline -Fe2O3 nanocubes and conveniently sortthe particle size and desired morphology through a facile,simple, economic, and straightforward strategy in a hydro-thermal method.
In the experiment reported herein, driven by the demandfor one pot facile hydrothermal synthesis of single crystalline-Fe2O3 nanocubes with well-defined morphology and scruti-nizing its size- and morphology-dependent properties, weemployed the chloride-based cation Fe3+ that was reactedwith the oleate functional group in the presence ofoleylamine (OAm). In the present work, it is noteworthy that:(i) the current synthetic method is very simple and straight-forward. The synthetic scheme involved only a one-step pro-cess that does not require high temperature or high pressure.(ii) It is particularly interesting to highlight that only the re-action time was varied in the range of 4 h to 12 h at a con-stant temperature, in contrast to other typical hydrothermalsynthesis of 24 h or a few days. The morphological evolutionof the nanostructures and their corresponding growth pro-cesses were able to be elucidated. Thereafter, detailed discus-sion based on the insightful analysis of the shape and struc-ture was also presented. (iii) We report on the shape- andsize-selective synthesis of single crystalline -Fe2O3 nano-cubes with good reproducibility without the use of additionalcapping agents or surfactants. Instead, we have used a mix-ture of solvents with OAm as one of the key elements to de-termine the final shape of single crystalline -Fe2O3nanocubes.46
Previously, many experimental and theoretical studies pri-marily focused on the size and morphology of the -Fe2O3nanostructures.4749 Since -Fe2O3 is well-known for its vari-ous applications. Hence, one of the most important aims ofthis paper was to present a study of the time-dependent mor-phological evolution of -Fe2O3 and its transformation from0D to 3D (cubic shape). Finally, the high quality singlecrystalline -Fe2O3 nanocubes are characterized by using vari-ous analytical tools to investigate the structural- and optical-properties of -Fe2O3 nanocubes in a much more detailedmanner. The as-obtained information is compared with thatof commercial -Fe2O3 nanoparticles to show the novelty ofthe as-synthesized product. In addition, the possible transfor-mation mechanism of the -Fe2O3 through a 1D 3Dmode was also proposed in the following sections. Thedetailed crystallographic-, structural- and optical-properties of
4722 | CrystEngComm, 2016, 18, 47204732 This journal is The Royal Society of Chemistry 2016
the as-prepared -Fe2O3 nanocubes presented are of funda-mental importance to comprehend the shape and growth ofsingle crystallinity of -Fe2O3 nanocubes and thus provide di-rect correlation between the shape and exposed facet-controlled properties to its applications in a future study.
All the chemicals used in the experiments were of analyticalgrade and no further purification was required. Potassiumoleate (C18H33KO2, Sigma Aldrich, 87%), ferric chloridehexahydrate (FeCl36H2O, Merck), Oleylamine (C18H35NH2,Sigma Aldrich, 70%) and absolute ethanol (C2H5OH, HmbGChemicals) were obtained. In this study, all the chemical re-actions were carried out in deionized water.
2.1 Synthesis of -Fe2O3 nanocubes
Hematite (-Fe2O3) cubic nanocrystals were synthesized usinga hydrothermal method. This experimental method of synthe-sis was carried out at different durations and the sampleswere collected every 2 h. The remaining factors were keptconstant while synthesizing the nanocrystals. Typically, amixture consisting of 3.33 mmol of potassium oleate wasdissolved in 5.34 mL ethanol with the addition of 1 mLoleylamine (OAm). Subsequently, the mixture was transferredinto a Teflon-lined stainless steel autoclave that has beenpre-loaded with 10.67 mL deionized water and 1.11 mmol ofFeCl36H2O aqueous solution. The resultant reagents werestirred continuously at room temperature for 2 hours until areddish solution was formed. Then, the autoclave was sealedand hydrothermally treated at 180 C for different heatingtimes (4 h, 6 h, 8 h, 10 h and 12 h). After the finite inductionperiod, the autoclave was left to cool down naturally. Theresulting dark red precipitate was collected by centrifugationand washed several times with ethanol. Finally, this synthe-sized material was dried in an oven at 55 C for 24 h.
The size, shape, morphology, chemical composition, struc-tural features and optical properties of the nanocrystals wereinvestigated by using different analytical characterizationtechniques. We systematically observed the time-dependentmorphological transformation of -Fe2O3 nanocrystal struc-ture via TEM images and SAED patterns with a JEOL JEM-2100F instrument operated at 200 kV. In order to observe theyield of -Fe2O3 nanocubes in large quantities, a field-emission scanning electron microscope (FESEM, HitachiSU8000 with the operation voltage of 2 kV) was employed. In-formation relating to the elemental mapping and elementalcomposition was studied by using energy dispersive X-rayanalysis (EDXA, Oxford Instrument). The bulk-crystallinityand phase identification of the samples were characterizedby X-ray powder diffractometer (XRD, PANalytical EMPY-REAN, 40 kV/35 mA with Cu K irradiation at = 1.5406 ).The scanning process covered an angular range from 10 to
80 with a scanning rate of 0.02 per minute. In addition, astudy of phonon vibration mode was conducted by means ofa Renishaw inVia Raman Microscope equipped with a LeicaDMLM microscope (objective lens of 50 (UV)). A 514 nmHeCd laser source was subjected onto the samples for 30 s at50 mW of laser power. The spectra were collected within therange of 150 to 800 cm1 by subjecting the samples to thebeam source at room temperature. The X-ray photoelectronspectroscopy (XPS) measurement was performed by usingsynchrotron radiation from photoemission spectroscopy(PES) beamline no. 3.2a at the Synchrotron Light Research In-stitute, Thailand in order to further study the chemical ele-ments and bonding of the single crystalline -Fe2O3 nano-cubes. The PES system was employed with a Thermo VGScientific CLAM2 electron spectrometer and functioned inthe conditions of m...