Nanomaterials: An Overview of Nanorods Synthesis and Optimization

4.1.1 Carbon nanorods

Carbon nanorods have attracted great interest from past few decades owing to their physical (particle size, shape, large surface area and greater pore size distribution) and chemical properties [16, 17, 18]. Nanorods made of carbon also known as “carbon nanorods” and “diamond nanorods.” Diamond nanorods have a crystalline structure like diamond with sp³ carbon hybridization. The yield and purity of synthesizing the carbon nanorods are strongly dependent on the composition of the inert atmosphere and its pressure. Generally, the carbon nanorods had better physicochemical properties after introducing different functionalities in the carbon nanorods pore surfaces. It will permit many applications, like in catalysis, water treatment, supercapacitors, and others. Carbon nanorods large applied as anodic material in batteries apart from their application like fillers [19] and high performance electrode materials in batteries [20, 21, 22, 23]. Till now, various synthesis methods have been proposed and it can be classified as “bottom up” (like synthesis from small molecules or colloidal solutions) or “top down” (like starting with bigger structures). The top-down method for synthesizing carbon nanorods including, simple chemical etching and electrochemical etching. Meanwhile, bottom-up approach including template assisted, metal assisted, hydrothermal route, vapor deposition (CVD), seed based and other synthesis in solution.

A recent finding was discussed on the recent advanced efforts in the preparation of carbon nanorods from metal-organic frameworks (MOFs) [24]. MOFs-a class of porous and crystalline material have attracted a great deal attention due to their fascinating architectures as well as their useful properties [25]. MOFs could be synthesized using both organic and inorganic components. Eventhough MOFs is well-established excellent porous material, yet the thermal transformation of MOFs into carbon materials accompanied by partial or complete collapse of their original morphology. Due to this reasons, the synthesis of nonhollow (solid) 1-D of carbon nanorods with moderate aspect ratio, high surface area and good performance capacitor electrodes is achieved by self-scarified and morphology-preserved thermal transformation of MOF-74 [24] (Figure 3).

The reaction of zinc nitrate and 2,5-dihydroxyterephthalic acid in N, N-dimethylformaide (DMF) by traditional hydrothermal method resulted in formation of microcrystalline MOF-74. The room-temperature reaction of those components in the presence of salicylic acid as a modulator led to the formation of rod-shaped MOF-74 (MOF-74-Rod, 30–60 nm wide, 200–500 long) as observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The addition of salicylic acid directed MOF growth in a rod-shape morphology by stabilizing the active metal sites on the MOF crystal surface [26]. The thermal transformation of MOF-74-Rod at 1000°C results in the formation of carbon nanorods. In comparison of MOF-74-Rod (377 m² g⁻¹), higher surface area of MOF-74 (411 m² g⁻¹) might be attributed to the perfect arrangement of MOF crystallites in the domain structure. The pore size distribution for MOF-74-Rid confirms the formation of micro (∼1.5 nm) and mesopores (∼6.5 nm), whereas MOF-74 shows the presence of microspores (∼1.2 nm) exclusively. The great different in pore size distribution attributed to the formation of voids resulted from lateral attachment of MOF nanorods. The MOF-74-Rod showed excellent capacitor performance with specific capacitance value of 164 F g-1 at sweep rate of 10 mVs⁻¹. Overall from this study, open up new avenues for efficient product of 1-D carbon material with promising applications in electrochemical devices.

4.1.2 ZnO nanorods

ZnO has a wide band-gap (3.37 eV at room temperature). ZnO is known to have wurtzite structure with lattice constant (a) 3.249 Å, (c) 5.207 Å. It has a large excitonic binding energy of 60 MeV, which is greater than the thermal energy at room temperature, makes it a promising candidate for applications in blue-UV light emission and room-temperature UV lasing. ZnO posed an excellent chemical and thermal stability and the electrical properties. Since Zno has lack of center symmetry, make it results in a piezoelectric effect, whereby a mechanical stress/strain could be transformed into electrical voltage and vice versa, due to the relative displacement of the cations and anions in the crystal [27]. Single crystal of ZnO exhibit significantly faster electron transport and greater mobility. The faster electron transport is a result of the high electron diffusion coefficients, which will provide significant advantages to device performance [28]. Since ZnO could emits at the near ultraviolet, has transparent conductivity and piezoelectric properties, thus, ZnO is an interesting material for semiconductor and laser devices, piezoelectric transducers, transparent electronics, surface acoustic wave devices, spin functional devices, and gas sensing. Overall, ZnO is an excellent material for sensor application attributed by its large surface to volume ratio that leads to the enhancement of it sensitivity, bio-safety and bio-compatibility. A recent research has demonstrated that creation of highly oriented and ordered array of ZnO nanostructures has greatly stimulate interest in development of novel devices [29]. The large surface area of nanorods makes ZnO attractive for gas and chemical sensing. High oriented array of ZnO nanorods (and nanowires) can be produced via various chemical, electrochemical and physical deposition techniques such as chemical vapor deposition (CVD) or metal organic CVD (MOCVD), vapor-liquid-solid (VLS) growth, electrochemical deposition (ED) and hydrothermal approaches.

Recently, a great deal of attention has been focused on the study of synthesizing the ZnO nanorods via VLS method. In this case, gold (Au) nanoparticles are used as catalyst in order to promoting the ZnO nanorods formation. Unfortunately, the are some apparent drawbacks in VLS growth technique. Generally, it required high growth temperature > 900°C in order to dissolving the Zn vapor into the Au catalyst simultaneously forming an alloy droplet. After saturated, Zn precipitates out from the droplet and further oxidized as ZnO nanorods grow. The other drawback from VLS growth method is that at the tips of ZnO nanorods there are always impurity particles that might be undesirable for fabrication. Due to this reason, the synthesizing ZnO nanorods via CVD and MOCVD were highlighted. The synthesis temperature used generally mild reaction temperature and high purity of ZnO nanorods could formed.

Apparently, the CVD process took place in a horizontal quartz tube placed in a rapid thermal furnace. Figure 4(a) shows a schematic illustration of the CVD furnace including a horizontal quartz tube of 1-in. A high-purity metallic granulated zinc (99.99%) was placed in an alumina boat which was then inserted at the end of quartz ampoule sealed at one end. Au nanoparticles are used as catalyst deposited on Chip B and C. Once the temperature went above the melting point of zinc metal (420°C), zinc would gradually vaporize to fill the quartz vial and then diffuse to Chip B and then to Chip C. The Au catalyst further formed liquid droplet and super saturated with Zn vapor. The nucleation growth of ZnO started with the arrival of oxygen gas. The ZnO will precipitate when the droplet reached a critical radius and continuously growth. Typically, ZnO synthesis was synthesized follow several steps. Initially, the quartz tube evacuated to 10–2 Torr, follow by purged using Ag gas to maintain a 1 atm ambient. The furnace temperature rapidly increased to 700°C under constant Ag flow and maintain within a period of time. Finally, the oxygen gas (O₂ mixed Ag) was then flown through quartz tube forcing a precipitate to form. As shown in Figure 4(c), shows prismatic hexagonal rods of ZnO grown area. The ZnO crystal continuously growth perpendicular from the surface on one single nanowires forming comb structure (Figure 4(d)). Thick ZnO needle can be found at the outer edge (Figure 4(e)). With sufficient oxygen concentration, wires with larger diameter are grown.

In the case of MOCVD techniques, in MOCVD generally, the use of organic precursor, such as Zn(C₂H₅)₂ and O₂ system, are involved. The ZnO films or nanorods were deposited on p-type silicon with (100) orientation. Figure 4(b) shows a schematic diagram of the MOCVD system. Mass flow controllers separately controlled the flow of Ar and O₂ gases and the gas flow ratio of Ar to O₂ was in the range of 1–2. The substrate temperature was varied as a process variable ranging from 250 to 500°C [30]. The deposition time was set to 10 min [30]. In this study the SEM image reveals that ZnO nanorods are directly grown on Si substrates (Figure 4(f)) and the diameter of ZnO nanorod ranges from 40 to 120 nm (Figure 4(g)). In summary, the uniform ZnO nanorods have successfully synthesized in bulk quantities directly on the Si substrate using the MOCVD technique.

Recently, several studies have demonstrates the growth of ZnO nanorods could be achieved molecular beam epitaxy (MBE). In MBE, the growth is performed under clean, low pressure condition and the reactants are very pure Zn metal and atomic O from a plasma generator [31]. In MBE system, the potential contamination is minimized [31] . The ZnO layers typically were grown on p-type silicon wafer Si(100) under conditions: substrate temperature 300–430°C, temperature of the Zn-Knudsen cell 300°C (Zn beads of purity 99.9999 were filled), pressure of the chamber during the growth was ∼(1–4) × 10^–4 mbar and oxygen plasma was generated [32]. The ZnO nanorods of reasonable quality could be deposited forming cored nanorod (Figure 5(a and b)). This cored nanorods could be produced using Mg-doping during MBE growth. Similarly, Heo et al. [33] report on catalyst-driven MBE of ZnO nanorods. The single ZnO nanorod growth is realized via nucleation on Ag films that are deposited on SiO₂-terminated Si substrate surface (Figure 5(c)). Growth occurs at substrate temperatures within range of 300–500°C. The nanorods are uniform cylinders exhibiting diameter of 15–40 nm (Figure 5(d)) and lengths in excess of 1 μm. Eventhough CVD, MOCVD and MBE such an attractive technique in developing ZnO nanorods but these growth techniques are complicated and growth temperatures used are high (435°C).

The hydrothermal method [34, 35] has attracted considerable attention due to its unique advantages-it is simple, low temperature (60–100°C), high yield, low cost, uncomplicated process, excellent morphology-well-defined structure and controllable process [36]. Particularly, chemical precursor solution involves in formation of ZnO nanorods via hydrothermal route is Zn salt and hexamethylenetetramine on Si substrates with a seed layer prepared from zinc acetate solution. Polyethyleneimine was added to the solution to increase the nanorod aspect ratio [37]. The growth temperature and the growth time were constantly kept at lower temperature < 100°C and under certain period of time. Recently, ZnO nanorods with hexagonal structure were synthesized via hydrothermal route by Polsongkram and co-workers using zinc nitrate [Zn(NO₃)₂ 6H₂O] that was mixed with hexamethylenetetramine (HMT) (C₆H₁₂N₄) solution and treated under temperature 60–95°C. It is evident that the at 95°C, the sample mainly consists of ZnO nanorods and most of them assembly into branched and urchin-like morphologies (Figure 6(a)). It was discovered that the hexagonal ZnO nanorods formed about 2 mm in length 100–150 nm in diameter. The nanorods grown larger (thick branched rods) when the temperature reduce to 75 and 60°C [35] (Figure 6(c and d)). This study also found that controlled growth of nanorods ranging from a thinner to a larger diameter can be realized by appropriate choice of the initial precursor concentration and deposition time. The hexagonal ZnO nanorods formation via hydrothermal method also in agreement with Phromyothin findings (Figure 6(d and e)) [38]. Similarly, this study also discovered that as the precursor concentration increased, the average diameter of ZnO nanorods will enlarge. It can be suggested that the precursor concentration provides the crucial role on the physical morphology and crystal growth direction of ZnO nanorods.

Last but not least, the ZnO nanorods also could be synthesized via ED method. ED method has many advantages including a low growth temperature, simple and low cost process without the need for vacuum systems for preparing ZnO nanorods with high crystallinity, being suited for scale-up and good electrical contact between the structures and the substrate [39]. However, in ED method when the ZnO nanorods were growth using electrochemical on transparent conducting oxides (TCOs, i.e. ITO and FTO), electrodes and a previously deposited ZnO seed layer are necessary to precisely control the morphology and aspect ratio of the as-grown ZnO nanostructures. In ED method the ZnO nanorods were electrodeposited from the zinc nitride aqueous solution in an electrode system. Typically, electrodepositions were conducted in a water bath at 80°C. Figure 6(f) presents the electrodeposited ZnO results, the result showed the SEM images of ZnO nanorods on predeposited PAN film [40]. The ZnO nanorods exhibited good vertical alignment, and with significant hexagonal cross section and a relatively uniform size with an average diameter of 180 nm.

4.1.3 Gold nanorods

Much attention has been given recently to gold nanorods (Au nanorods), mainly due to their applications in biomedicine. Gold nanorods show two absorption bands, known as surface plasmon resonance (SPR) bands, called the TSPR (transverse) in the visible and LSPR (longitudinal) in the near infrared (NIR) region [41]. This last one is useful for medical applications because NIR radiation is the one that penetrates the most in living tissues. The absorbed radiation is converted into heat, thus showing promise for cancer treatment. Also, these nanorods have localized surface plasmon resonances (LSPRs) that allow for unique scientific and technical applications [42]. In particular, the synthesis of well-defined size and shapes of Au nanorods has attracted much attention due to its importance in electronic and optical properties of these nanomaterials. The longitudinal bands of Au nanorods can be tuned by changing their aspect ratio, simultaneously make it possible to gain absorption bands at the desired wavelength in the NIR. Small change in aspect ratio will result in drastic change in the NIR absorption wavelength. The Au nanorods could be synthesized via two general growth approaches, which are bottom-up and top-down methods. For bottom-up methods, Au nanorods are generated through nucleation in aqueous solutions and subsequent overgrowth, where Au salts are usually used to provide the Au source through reduction. Particularly, bottom-up method including wet-chemical, electrochemical, sonochemical, solvothermal, microwave-assisted and photochemical reduction technique. All of these method involving the use of reduced aqueous solvated Au salts by various reducing agents, such as sodium borohydride, ascorbic acid, and small Au clusters, under different external stimuli (triggering the reduction of Au salt). The length of Au nanorods could be elongated with the use of template, it serves to confine the growth along one direction during the reduction.

The electrochemical method was the first technique for developing the Au nanorods. Briefly, the Au and Pt were used as the anode and cathode, respectively. These electrodes will be immersed in an electrolytic solution containing the cationic surfactant such as hexadecyltrimethylammonium bromide (CTAB) and co-surfactant. The CTAB works as supporting electrolyte and stabilizer (preventing aggregation of the nanorods), and CTAB induces the formation of rods. The length of the nanorods is determined by the presence of a silver plate in the solution. The silver metals react with the Au ions generated by the dissolution of the anode. The researchers found that the amount of dissolved silver and the concentration of Ag⁺ ions determined the length of the nanorods [25]. The larger the area of silver plate, the higher the amount of Ag+ ion species formed and the higher the speed of silver will be released and thus, the longer the Au nanorods formed [43].

Based on previous literature for Au nanorods formation via bottom-up method, seed mediated growth has been by far the most efficient and popular approach [44]. This method utilizes “soft templates” for growing Au (Figure 7), which was developed by Murphy and El-Sayed studies [42]. Highly yield monodisperse Au nanorods with greater uniformity could be developed via this method. Typically, small Au nanoparticles seed of ∼1.5 nm is initially prepared by reducing chloroauric acid with borohydride in an aqueous CTAB solution [42]. The seed solution will be mixed with growth solution containing metal salt (weak reducing agent) such as ascorbic acid and a surfactant-directing agent CTAB. The CTAB will absorbed onto Au nanorods forming a bilayer. It is suggested that aspect ratio of Au nanorods can be controlled by ratio metal see/metal salt in growth solution. According to former study, the CTAB will bind to the crystallographic faces of Au existing along the sides of pentahedrally twinned rods, as compared to the faces at the tip. The seed-mediated growth technique is the presence of CTAB is one of the most widely used and the yields of the nanorods from the seed-mediated growth method can be as high as 99%. It have been reported that the size and shape of Au nanorods could be tailored by adjusting the growth condition such as the pH of growth solution, composition of surfactant, amount of the reagent, growth temperature and structure of the seed in the seed-mediated growth process. Interestingly, recent study demonstrated high yield and greater uniformity of Au nanorods could be obtained via seed-mediated growth through the use of aromatic additive to CTAB [45]. Figure 7(a and c) showed TEM images of Au nanorods synthesized with 0.0126 M sodium 3-methylsalicylate (additive) present in the growth solution. The nanorods obtained have an average diameter of 14.0 (1.0 nm and a length of 33.0 ± 2.5 nm. On the other hand, slightly longer nanorods are made when 0.010 M sodium salicylate is used as the additive (Figure 7(b and d)).

Eventhough the bottom-up method results in excellent monodisperse Au nanorods with small diameter and high uniformity, yet them suffer some drawback where typically, selective placement of Au nanorods at desired locations on substrates by the bottom-up methods has been very difficult owing to the random nature of the reduction of Au ions and the deposition of Au atoms in reaction solutions. Moreover, the shape and size of Au nanorods also varied from different synthesis batches. This will affect their optical and catalytic properties and applications. Last but not least, bottom-up method suffers in placing Au nanorods into large-area, ordered arrays. Due to these reasons, top-down approaches gained interest.

It is well-established that top-down method could promote high production homogeneous Au nanorods with controlled particle geometries and regular inter-particle arrangements, which is valuable for quantitative characterization as well as device applications. In top-down methods, Au nanorods are obtained through a combination of different physical lithography processes and Au deposition [42]. Particularly, there are two technique used for top-down method in synthesizing the Au nanorods. First method is through the removal of Au from predeposited Au films using ion beam or etching techniques. Second method is by employing the lithography techniques to create mask. Au layer then deposited on the substrate which is covered by the mask via physical method: thermal, electron-beam evaporation or sputtering. The synthesized Au nanorods further obtained from lift-off process. Generally, the size of Au nanorods obtained via top-down method is limited by the resolution of lithography method. Interestingly, recent study by Koh and co-worker reported that state-of-the-art electron beam lithography system able to produce the size of Au nanorods within diameter ranging from ∼10 to >100 nm [46]. The Au nanorods were fabricated on 30-nm-thick silicon-nitride (SiN) membranes (Figure 8(a)). High-resolution TEM images of individual nanostructures of Au nanorods in Figure 8(b) reveal that the Au nanostructures were polycrystalline in nature. The polycrystallinity of the metal structures could potentially be a drawback of lithographically defined metal structures, as chemically synthesized metal nanoparticles can be a single crystal when synthesized in certain conditions. Eventhough, Au nanorods successfully synthesized via top-down method yet this method is time consuming and costly makes them impractical for fabrication of Au nanorods in bigger scales. Furthermore, Au nanorods obtained from vacuum deposition techniques also can degrade their plasmonic properties due to the electron scattering at the grain boundaries. Due to these reason had limit their usage in device application and simultaneously make the top-down method to be unattractive in fundamental research.

4.1.4 Magnetic nanorods

Nanostructured iron oxide magnetite (Fe₃O₄) behaves supermagnetic and widely used in the biomedical field as well as device application. Generally, the magnetic nanoparticle could be utilized as nanoadsorbents, cancer diagnostic and treatment, contrast agent in magnetic resonance imaging (MRI), etc. Meanwhile for 1-D magnetite extremely important in building block for nanodevices. It has been found that size and shape of magnetite nanoparticles play key role in controlling the corresponding properties [47]. The magnetite nanoparticles could be synthesis via aqueous co-precipitation, magnetic field induction, CVD, template mediated, etc. [48, 49, 50]. The aqueous co-precipitation of Fe²⁺ and Fe³⁺ by a base, usually sodium hydroxide or aqueous ammonia, is the well-known method which is usually carried out for synthesizing the magnetite nanoparticle [51]. This method is the most scalable chemical synthesis routes which results in iron oxide nanospherical crystal. However, there is a study also found that innovative modification in co-precipitation technique through incorporation of special aqueous solution will lead to the formation of nanocubes or nanorods iron oxide particle’s. This was in agreement with Khalil finding’s [52].

Recently, considerable attention has been drawn to production of 1-D magnetite nanorods due to their high surface to volume ration and superior properties. Due to the high aspect ratio, magnetic nanorods have the high values of coercivity and produce a lot of heat in high frequency magnetic field, which offer longer blood circulation times, stronger interaction with tumors, enhanced retention at tumor sites and improved targeting efficiency [47, 53]. All of these reasons stimulate their making in excellent candidates as targeting pharmaceutical carrier or MRI contrast agents. For instance, iron oxide nanocubes including nanorods structure with a length larger than 100 nm could be achieved via thermal decomposition, wet chemical, hydrothermal, template mediated, solvothermal, hydrolysis and sol-gel.

Thermal decomposition method involved chemical decomposition at high temperature, lead to the breaking of the chemical bonds. The thermal decomposition method for synthesis iron oxide nanorods involve metal-organic compound. The obtained nanorods generally with diameter and length within range of 50–100 nm. This method typically led large nanorods structure due to the annealing by high reaction temperature. It is in agreement with Chen et al. finding reported that with the increasing reaction temperature, the aspect ratio of the products decreases to some extent, thus no any rod-like particles produced when high temperatures synthesis method is used [54]. Meanwhile, eventhough the co-precipitation method is a conventional method for synthesizing iron oxide nanorods, yet this method often uses trioctylphosphine (TOP), tributylphosphine (TBP), trioctylphosphine oxide (TOPO) or oleylamine (OA) and other long chain amines as solvents and capping agents in order to prevent the uncontrolled precipitation. Since this nanoparticle used for biomedical application, hence the nanoparticles of iron oxides should be a nontoxic. The usage of nontoxic capping agent and stabilizer eliminates the use of toxic and expensive chemical such as TOP, TBP or amines.

In the past years, the wet chemical synthesis of iron oxide nanorods via one step wet chemical method have been reported by several groups [54, 55]. Chen’s group reported in the one-step synthesis method and a surfactant, polyethylene glycol (PEG) was used as template and ferrous ammonia sulfate was use as precursor [54]. This study agreed that a formation of iron oxide nanorods can be achieved at longer retention synthesis time and adjusting the diffusion of ammonia by implementation a suitable ratio between the rates of deposition and oxidation of ferrous ions. These were in agreement with TEM image the iron oxide synthesize at from 2 to 10 h and it is reveals that the iron oxide nanorods could be obtained upon extended reaction time. By adjusting pressure of ammonia, i.e., adjusting the concentration of aqueous ammonia in the right flask also results in formation of pure magnetite phase. Further study on synthesizing iron oxide nanorods via wet chemical method were also reported by Ahmed and co-worker [56]. The obtained nanoparticles were rod shaped and consisted mainly of maghemite (ϒ-Fe₂O₃) phase. The nanoparticles also appear superparamagnetic behavior under room temperature.

Solvothermal method is an effective method for producing iron oxide nanorods. Si et al. present a method for obtaining the iron oxide nanorods via solvothermal method, and product showed formation of iron oxide nanorods with diameter size within range of 58–250 nm and width from 8 to 64 nm (Figure 9(a and b)) [57]. The iron oxide nanorods obtained exhibit uniform size and greater dispersion in nonpolar solvent (cyclohexane). It also revealed the single crystalline nature of nanorods is successfully produced upon synthesis process. The effectiveness of solvothermal method in producing iron oxide nanorods was in agreement Sun and co-worker finding, which reported that Fe₃O₄ single crystal nanorods with a uniform length between 64 and 140 nm can be prepared using Fe(CO)₅ in the presence of oleic acid through a solvothermal process [58].

Recent by Kloust et al. also discovered that the iron oxide nanorods can be produced directly from template mediated approach [59]. The iron oxide nanorods synthesis directly from iron oleate in one-step procedure. The iron oleate is used as a precursor. In this method, the iron oxide nanodot string together in a row to form rods-structure (Figure 9(c)). Based on the TEM image the formed iron nanorods exhibit a mean length of approximately 24 nm and mean diameter 2.5 nm with an associated aspect ratio of 10 (Figure 9(d)). The iron oxide nanorods produced particularly small and thin nanorods. The nanodot string combination mechanism have been confirmed in TEM micrograph, which shows the alignment of single dot characteristic. The high resolution TEM image of iron oxide nanorods (Figure 9(e)) confirms this characteristic and show twisted crystal-orientations of single nanocrystal. Further magnetic characterization reveals that the iron oxide nanorods synthesized via template mediated approach posed super-paramagnetic behavior. In the blocked state the nanorods exhibit a magnetic easy axis parallel to the long axis of nanorods due to the enhancement shape anisotropy. Kloust et al. [59] also presented a method for preparation of iron oxide nanorods with a template approach, where iron oxide nanodots string together. The procedure uses iron oleate as a precursor and is a one-step synthesis. A precise tuning of the width of the nanorods between 1 and 6 nm was realized.

Hydrothermal method might flexible method for synthesizing iron oxide nanorods. Thus, several procedures have been developed [48, 49, 50]. Hydrothermal methods rely on the ability of water at elevated pressures and temperatures to hydrolyze and dehydrate metal salts, and the very low solubility of the resulting iron oxide in water at these conditions to generate supersaturation [60]. Extremely high supersaturation could be achieved in the reaction process attributed by the lower solubility of metal hydroxides and oxides, thus very fine crystals will form. Similar case with thermal decomposition methods, the hydrothermal system also involves high temperature synthesis condition. Thus, the size of the nanoparticles had a larger particle (if the hydrothermal method proceed under supercritical temperature (<350°C)). This was due to the promotion of crystal growth that results from the dissolution and precipitation process in sub-critical water. The particle size also increased with the increase in reaction pressure in supercritical water [61]. Liang and co-workers report the variation of crystallite and particles size of iron oxide at temperature 250 and 350°C; the crystallite size of iron oxide increased significantly as the temperature was raised from 250 to 350°C. This phenomenon is due to the nucleation process which occurred as the monomer concentration reached the saturation point [62]. Interestingly, the size of particles will reduce and become smaller if the hydrothermal method proceeded under temperature > 380°C (above critical temperature). This is probably due to a rather low solvent power of supercritical water and an extremely high hydrolysis rate of iron salt in supercritical water. Therefore, very high super-saturation is achieved in which results in fine iron oxide crystals nucleating in situ immediately. It is in agreement with Arai et al. study [61]. Eventhough hydrothermal method considered effective in synthesizing iron oxide nanoparticles, yet the particles shape difficult to control in order to form pure rod-like structure.

Other method that effective in synthesizing the iron oxide nanorods is sol-gel method. Piao et al. reported on wrap-bake-peel process for nanostructural transformation from β-FeOOH nanorods to biocompatible iron oxide nanocapsules [63]. This process involves silica coating, heat treatment and lastly the removal of silica layer, in order to transform the phases and structures of nanostructured materials while preserving their nanostructural characteristic. Water dispersible hollow iron oxide nanocapsules were obtained by applying the wrap-bake-peel process to β-FeOOH nanoparticles. The synthesized magnetite nanocapsules could be successfully used not only as a drug-delivery vehicle, but also as a T2 MRI contrast agent.