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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: firstname.lastname@example.org,
email@example.comDepartment 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 m