Hydrothermal growth mechanism of α-Fe₂O₃ nanorods derived by near in situ analysis

Abstract

The hydrothermal growth mechanism of α-Fe₂O₃ nanorods has been investigated using a novel valve-assisted pressure autoclave. This approach has facilitated the rapid quenching of hydrothermal suspensions into liquid nitrogen, providing ‘snapshots’ representative of the near in situ physical state of the synthesis reaction products as a function of known temperature. Examination of the acquired samples using complementary characterisation techniques of transmission electron microscopy, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy (FT-IR) has provided fundamental insight into the anisotropic crystal growth mechanism of the lenticular α-Fe₂O₃ nanorods. An intermediate β-FeOOH phase was observed to precipitate alongside small primary α-Fe₂O₃ nanoparticles. Dissolution of the β-FeOOH phase with increasing temperature, in accordance with Ostwald's rule of stages, led to the release of Fe³⁺ anions back into solution to supply the growth of α-Fe₂O₃ nanoparticles, which in turn coalesced to form lenticular α-Fe₂O₃ nanorods. The critical role of the PO₄³⁻ surfactant on mediating the lenticular shape of the α-Fe₂O₃ nanorods is emphasised. Strong phosphate anion absorption on α-Fe₂O₃ crystal surfaces stabilised the primary α-Fe₂O₃ nanoparticle size to < 10 nm. FT-IR investigation of the quenched reaction products provided evidence for PO₄³⁻ absorption on the α-Fe₂O₃ nanoparticles in the form of mono or bi-dentate (bridging) surface complexes on surfaces normal and parallel to the crystallographic α-Fe₂O₃ c-axis, respectively. Mono-dentate PO₄³⁻ absorption is considered weaker and hence easily displaced during growth, as compared to absorbed PO₄³⁻ bi-dentate species, which implies the α-Fe₂O₃ c-planes are favoured for the oriented attachment of primary α-Fe₂O₃ nanoparticles, resulting in the development of filamentary features which act as the basis of growth, defining the shape of the lenticular α-Fe₂O₃ nanorods.

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1. Introduction

Nanostructured materials have attracted considerable attention because of their unique functional properties.^1–6 Weakly-ferromagnetic α-Fe₂O₃ (hematite) is of particular interest as a cheap, environmentally friendly and thermodynamically stable iron oxide, and one-dimensional (1D) α-Fe₂O₃ nanorods have been studied for a wide range of applications because their magnetic properties are greatly dependent on nanorod size and shape.^7–10 1D nanorods are capable of exhibiting much higher coercivities than their isotropic counterparts because of the effect of shape anisotropy.¹¹

To date, α-Fe₂O₃ nanostructures have been produced using a variety of techniques including sol–gel processing,¹² microemulsion,¹³ forced hydrolysis,¹⁴ hydrothermal synthesis (HS)¹⁵ and chemical precipitation.¹⁶ In particular, HS offers effective control over the size and shape of nanoparticles at relatively low reaction temperatures and short reaction times, providing for well-crystallized reaction products with high homogeneity and definite composition.¹⁷ Indeed, the technique of continuous and high throughput HS has become a viable method for the commercial synthesis of nanoparticles.^18–21 However, there is still much speculation concerning the fundamental mechanisms of hydrothermal reaction and nanoparticle growth. The use of reaction vessels under conditions of high temperature and pressure generally creates an aspect of inaccessibility which limits direct investigation. Indeed, even the exception of in situ synchrotron and energy dispersive X-ray diffraction methods are sensitive only to bulk crystal phase transformations.^22–24In situ X-ray absorption spectroscopy, for example, allows time-resolved experiments under specific reaction conditions to provide the structural and electronic properties around a specific atom,^25,26 but does not offer direct information on the morphological development and kinetics involved in the growth of nanostructures. Consequently, the analysis of HS nanoparticles on the localised scale has been restricted to post-synthesis reaction products isolated following cool down under equilibrium conditions, with possibility of loss of in situ constituent evidence demonstrating the specific mechanistics of growth.

Aqueous iron (III) chloride (FeCl₃) solution is well established as a simple precursor for the formation of monodispersed α-Fe₂O₃ nanoparticles,^27,28 and needle-shaped β-FeOOH nanoparticles are known to form as an intermediate phase which gradually diminishes as the α-Fe₂O₃ nanoparticles develop.^29–31 Further, a small addition of phosphate anions has been shown to mediate the anisotropic growth of α-Fe₂O₃, leading to the development of lenticular nanorods.^31–33Ex situ investigations of the phase transformation of β-FeOOH to α-Fe₂O₃ to date, along with α-Fe₂O₃ shape control within phosphate environments, have provided valuable insights into the α-Fe₂O₃ nanorod growth mechanism.³⁴ However, improved understanding of the in situ mechanistics of α-Fe₂O₃ nanorod growth is required in order to control fully the functional properties of these 1D nanostructures.

In this context, we have reported previously on the development of a novel, valve-assisted, Teflon-lined, hydrothermal pressure vessel which allows for the rapid quenching of hydrothermal products.³⁵ It is considered that the reaction products obtained from this autoclave are closely representative of the in situ physical state of the developing iron oxide nanostructures due to the large thermal gradient experienced during quenching. This novel investigative approach restricts possible atom/ion transport, thereby locking-in the morphology of the nanostructures to reveal the intricate details of the hydrothermal growth. Indeed, our initial experiments have demonstrated the viability of this ‘snapshot’ approach to provide detailed insight into the localised development of lenticular α-Fe₂O₃ nanorods, as a function of reaction time.³⁵ We now report on a more detailed investigation of the development of 1D α-Fe₂O₃ nanostructures, as a function of reaction temperature, using this valve-assisted hydrothermal pressure vessel, recognising that thorough understanding of the temperature response of each reaction vessel is needed to fully appraise the growth. It is considered that detailed analysis of near in situ hydrothermal reaction solutions, acquired at known reaction temperatures, provides for better understanding of the development process of α-Fe₂O₃ nanorods. In particular, emphasis is given to the critical role of the PO₄³⁻ surfactant mediating their lenticular shape.

2. Experimental

A novel 50 ml valve-assisted Teflon-lined steel autoclave was used to investigate the in situ hydrothermal growth of α-Fe₂O₃ nanorods, the construction of which has been described previously.³⁵ Nano-structured iron oxide was prepared through the hydrothermal reaction of 0.1 ml FeCl₃ aqueous solution (45% pure FeCl₃; Riedel-de Haen, Germany), further diluted in 20 ml of distilled water and mixed with 1.5 mg of ammonium dihydrogen-phosphate (99.999% NH₄H₂PO₄; Sigma-Aldrich, UK), then sealed and inserted into a temperature controlled furnace at the reaction temperature. The time taken for the hydrothermal reaction solution to reach the furnace temperature (Table 1 - samples labelled S100 to S200, for simplicity) was monitored using an integrated thermal couple (Fig. 1). This demonstrated that the autoclave, in this instance, required a dwell time of 80 min to achieve the desired hydrothermal reaction temperature, whilst the heating rate was dependent on the furnace temperature. The autoclave was then removed from the furnace at the reaction temperature and transferred immediately to a tripod support, for stability, where the valve was opened and closed, as quickly as practically possible, for the release of sufficient hydrothermal product suspension into liquid nitrogen for the purpose of rapid quenching, providing ‘snapshots’ of the α-Fe₂O₃ nanorod growth, as previously described.³⁵ It is recognised that the cooling rate is dependent on the reaction solution pressure and thus the associated solution expulsion rate from the valve. At low pressures, it is considered that the quenching time might be in the range of tenths of a second, whilst at high pressures, the cooling rate is thought to be significantly higher. The resultant suspension comprised frozen droplets, from a few tens of micrometres up to a few millimetres in diameter, with pale orange to reddish/brown appearance.

Table 1 ‘Snapshot’ samples investigated as a function of increasing temperature and a summary of the product phases, morphologies and sizes

Snapshot Sample	Temperature/°C	Phase and morphology	Size (length/width)
S100	100	β-FeOOH nanorods Small α-Fe2O3 nanoparticles	< 50/15 nm < 10 nm in diameter
S120	120	β-FeOOH nanorods Small α-Fe2O3 nanoparticles	< 50/15 nm < 10 nm in diameter
S140	140	β-FeOOH nanorods Small α-Fe2O3 nanoparticles Lenticular α-Fe2O3 nanorods	< 50/15 nm < 10 nm in diameter ∼ 150/50 nm
S160	160	Few β-FeOOH nanorods Small α-Fe2O3 nanoparticles Lenticular α-Fe2O3 nanorods	< 40/15 nm < 10 nm in diameter ∼ 350/60 nm
S180	180	Small α-Fe2O3 nanoparticles Lenticular α-Fe2O3 nanorods	< 10 nm in diameter ∼ 420/65 nm
S200	200	Lenticular α-Fe2O3 nanorods	∼ 450/80 nm

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Fig. 1 Hydrothermal reaction solution temperature plotted as a function of reaction time.

Tiny frozen droplets of the synthesised reaction product were deposited straight onto lacey carbon/copper mesh support grids (Agar Scientific Ltd, UK) and allowed to melt at room temperature prior to transmission electron microscopy (TEM) investigation. Conventional diffraction contrast bright field (BF) and phase contrast imaging were performed using Jeol 2000fx and 2100F electron microscopes. Selected area electron diffraction (SAED) patterns allowed for phase identification and an appraisal of the relationship between nanorod morphology and crystallographic orientation. X-ray photoelectron spectroscopy (XPS) (Kratos AXISULTRA configured with a monochromated Al_K X-ray source) was utilized to investigate the surface chemistries of the nanoparticles. Further, Fourier transform infrared (FT-IR) transmittance spectroscopy (Bruker Tensor, Germany; OPUS Spectroscopy Software) provided information on the covalent bonding present. Frozen droplets were allowed to melt in a pipette at room temperature and deposited directly onto the FT-IR diamond sensor, where it was dried at 80 °C using the heating stage and subsequently clamped. CasaXPS was used to analyze the XPS and FT-IR spectra.

3. Results

Use of the valve-assisted pressure vessel enabled near in situ investigation of the hydrothermal processing of aqueous FeCl₃ solution, for the synthesis of either β-FeOOH or α-Fe₂O₃ nanostructures, or both, depending on the HS reaction temperature (or time). In the present study, the valve-assisted pressure vessel was used to obtain snapshots of the synthesis after 80 min of processing, after the HS reaction solution had attained the target (furnace) temperature, the evidence for which is now presented in detail.

The bright field, diffraction contrast TEM images of Fig. 2a-f illustrate the varied β-FeOOH and α-Fe₂O₃ reaction products, as a function of increasing HS temperature (Samples S100 to S200). The phases present were identified from associated SAED patterns (Fig. 2a-f inset and summarised in Table 1). Sample S100 (Fig. 2a) showed the presence of β-FeOOH needle-shaped nanorods (< 50 nm long, 15 nm wide) and small α-Fe₂O₃ nanoparticles (< 10 nm in diameter). The same phases and particle morphologies were observed in Sample S120 (Fig. 2b), with the presence of slightly more of the small α-Fe₂O₃ nanoparticles. Conversely, Sample S140 (Fig. 2c) exhibited additional darker α-Fe₂O₃ nanorods (∼ 200 long, ∼ 50nm wide, as arrowed) amongst the previously observed β-FeOOH and α-Fe₂O₃ nanostructures. Further, Sample S160 (Fig. 2d) demonstrated an increased presence of the small isotropic α-Fe₂O₃ nanoparticles (< 10 nm in diameter), along with the development of larger α-Fe₂O₃ lenticular nanorods (∼ 350 nm long, ∼ 60 nm wide) and a correspondingly marked reduction in the presence of the intermediate β-FeOOH phase. The proportion and size of α-Fe₂O₃ nanorods (∼ 420 nm long, ∼ 65 nm wide) was even greater in Sample S180 (Fig. 2e), without any noticeable evidence for the presence of the intermediate β-FeOOH phase amongst the remaining small α-Fe₂O₃ nanoparticles. Sample S200 exhibited the largest, most well-defined, crystalline lenticular α-Fe₂O₃ nanorods (∼ 450 nm long, ∼ 80 nm wide) (Fig. 2f).

Fig. 2 Bright field TEM images of ‘quenched’ hydrothermal products after 80 min of processing, having achieved temperatures of (a) 100, (b) 120, (c) 140, (d) 160, (e) 180 and (f) 200 °C, respectively (Samples S100 to S200), with corresponding SAED patterns (inset – (a) and (f) are indexed to β-FeOOH and α-Fe₂O₃, respectively).

Phase contrast imaging of Sample S100 revealed details of the initial development of β-FeOOH nanostructures, as identified by characteristic lattice fringes corresponding to {110} and {200} planes (Fig. 3a and 3b, respectively). An embryonic β-FeOOH nanoparticle is shown in Fig. 3a, whilst Fig. 3b is representative of the first stages of development of a β-FeOOH nanorod, with characteristic {110} planes (d₁₁₀ = 7.47Å) lying parallel to the nanorod growth axis (Fig. 3b).

Fig. 3 Phase contrast TEM images of the hydrothermal products synthesised at 100 °C (Sample S100). (a) Small β-FeOOH nanoparticle (∼ 15 nm in diameter); and (b) small developing β-FeOOH nanorod (∼ 50 nm long, ∼ 15 nm wide); both identified by lattice fringes.

The bright field and phase contrast TEM images of Fig. 4 present details of the initially formed α-Fe₂O₃ nanoparticles, as identified in Sample S160. Lattice fringes consistent with the development of crystalline α-Fe₂O₃ were identified (Fig. 4d), and confirmed by associated Fast Fourier Transform (FFT) patterns (Fig. 4e). The initially formed α-Fe₂O₃ nanoparticles were all isotropic in shape and < 10 nm in diameter.

Fig. 4 Bright field and phase contrast TEM images of the first stages of development of α-Fe₂O₃ nanoparticles, synthesised at 160 °C (Sample S160). (a) Small α-Fe₂O₃ nanoparticles (< 10 nm in diameter); (b) & (c) small α-Fe₂O₃ nanoparticles on the edge of the lacey carbon support; (d) an individual α-Fe₂O₃ nanoparticle identified by lattice fringes and (e) corresponding FFT.

TEM investigation of Samples S160 and S180 (Fig. 5a-d) demonstrated the close association between the initially formed α-Fe₂O₃ nanoparticles and the development of single crystalline α-Fe₂O₃ nanorods. Fig. 5a shows a small crystalline α-Fe₂O₃ nanorod (∼ 40 nm long, ∼ 12 nm wide), as identified by lattice fringe spacings and associated FFT. Fig. 5b illustrates a slightly larger α-Fe₂O₃ nanorod (∼ 120 nm long, ∼ 25 nm wide), with an irregular surface. The developing tips of such α-Fe₂O₃ nanorods exhibited filamentary features, comprising α-Fe₂O₃ nanoparticles of similar size to those shown in Fig. 4, with lattice fringes aligned with the nanorod bulk (Fig. 5c, inset). Associated SAED patterns (Fig. 5c inset) demonstrated the α-Fe₂O₃ nanorod growth axis to be parallel to <006>. It is considered that the TEM image of Fig. 5d reveals details of the initial stages of attachment of crystalline α-Fe₂O₃ nanoparticles to a developing α-Fe₂O₃ nanorod, prior to their crystallographic alignment with the bulk nanorod.

Fig. 5 Bright field and phase contrast TEM images of the hydrothermal products processed at (a–c) 160 °C and (d) 180 °C, respectively. (a) Initially formed α-Fe₂O₃ nanorod (∼ 40 nm long, ∼ 12 nm wide), as identified by lattice fringes and associated FFT (both inset); (b) α-Fe₂O₃ nanorod (∼ 120 nm long, ∼ 25 nm wide) with corresponding SAED pattern (inset); (c) tip of an α-Fe₂O₃ nanorod, as identified by lattice fringes and SAED pattern (both inset), exhibiting filamentary features crystallographically aligned with the bulk nanorod (inset); and (d) tip of a developing α-Fe₂O₃ nanorod, as identified by lattice fringes (inset), with an irregular surface and uneven distribution of α-Fe₂O₃ nanoparticles attached to the tip (inset).

Complementary XPS data from samples S100 and S160, showing details of Fe 2p and P 2p peaks are presented in Fig. 6a and 6b, respectively (with additional survey scans given in the Supplementary materials). In our previous investigation,³⁶ it was reported that Fe 2p and P 2p signatures from iron oxide and iron hydroxide grown by HS in a phosphate environment were nearly identical, limiting ease of discrimination between the surfaces of these phases using the technique of XPS. However, subtle differences in the near surface chemistries of these β-FeOOH and α-Fe₂O₃ nanostructures are revealed by the present data set.

Fig. 6 XPS data showing (a) Fe 2p and (b) P 2p signatures from Samples S100, S160 & S200.

Fig. 6a shows Fe 2p_3/2 binding energy peaks at 711.8 eV and 711.0 eV for samples S100 and S160, consistent with the presence of β-FeOOH and α-Fe₂O₃, respectively.³⁷ Satellite peaks characteristic of Fe³⁺ were evident within both spectra, with differences in the binding energies between satellite and Fe 2p_3/2 peaks being 7.60 eV and 4.50 eV for α-Fe₂O₃ and β-FeOOH, respectively. This observation is consistent with a recent report of an 8.0 eV separation between satellite peak and the Fe 2p_3/2 binding energy for bulk α-Fe₂O₃.³⁸ Further, Fig. 6b showing P 2p peaks centred at 134.0 eV, 133.0 eV and 133.1 eV (Samples S100, 160 and S200, respectively) is consistent with the presence of phosphate species on the surfaces of both β-FeOOH and α-Fe₂O₃ nanostructures.^39,40

The nature of the covalent bonding of the surface phosphate species was investigated using FT-IR spectroscopy. Phosphate stretching modes⁴¹ were identified typically in the range of 900–1200 cm⁻¹. Fig. 7a presents FT-IR spectra obtained from Samples S100, S160 and S200, as well as an additional spectrum from acetone cleaned S200 (labelled S200C). It is clear that the signal from Sample 200C is relatively small compared to Samples S100, S160 and S200. For this reason, the signal from the original snapshot samples can be assigned to emission from surface species, as opposed to signal from the crystal bulk. Deconvolution of the S100 and S160 spectra into the individual spectral constituents was performed using CasaXPS software (Fig. 7b and 7c/d, respectively). Sample S100 demonstrated peaks at 1111, 1056 and 987 cm⁻¹ (Fig. 7b) whilst Sample S160 showed peaks at 898, 934, 970, 1004, 1036, 1071 and 1149 cm⁻¹ (Fig. 7d).

Fig. 7 (a) FT-IR spectra of phosphate species from Samples S100, S160, S200 and S200C; (b) deconvolution of S100 showing labelled individual spectral constituents; (c) deconvolution of S160 showing individual spectral constituents; and (d) labelled individual peaks (Sample S160).

4. Discussion

4.1 Overview

It is considered that sequential snapshot characterisation of the hydrothermal reaction products as a function of processing temperature allows for direct realization of the α-Fe₂O₃ nanorod growth mechanism. In this context, the present set of experiments chart the development of β-FeOOH and α-Fe₂O₃ reaction product phase and morphology as a function of stabilised temperature, attained using a 50 ml valve-assisted autoclave after 80 min of processing. Two considerations, however, should be noted. Firstly, at the high temperatures and pressures associated with HS, there might be some possibility of parasitic crystallisation and explosive nucleation caused by high supersaturation during quenching, as well as contamination artefacts. Secondly, it is recognised that present set of experiments provides a series of steady-state observations as a function of known temperature, as distinct from a time dependent analysis of the development of reaction products, as previously described.³⁵ A lack of knowledge of the precise temperature during the time dependent heating stage in our earlier work prohibited direct association of HS temperature with nanostructure development.³⁵ Nevertheless, the overall growth sequence elucidated using the present snapshot approach at known temperature, as reported here, is found to match the trends of our previously reported HS process-map for a larger, 200ml, autoclave used for the production of lenticular α-Fe₂O₃ nanorods as a function of temperature in the presence of a phosphate surfactant, after 2 h of processing time.³⁶ Indeed, it is worth emphasising that the original process map explored a wider range of processing conditions, i.e. reaction time; temperature and phosphate concentration, and this focused our attention on the present series of snapshot experiments at known temperature, for the purpose of gaining improved understanding of the hydrothermal growth mechanism. Accordingly, embryonic β-FeOOH nanoparticles and nanorods, by way of intermediate phase (Sample S100), led to the development of small (< 10 nm) α-Fe₂O₃ nanoparticles (Sample S120) which increased in proportion along with the development of small crystalline α-Fe₂O₃ nanorods (Sample S140). Maximisation of the quantity of small α-Fe₂O₃ nanoparticles and corresponding reduction in the proportion of β-FeOOH nanorods with increasing processing temperature (Sample S160) led to further increase in the quantity and size of α-Fe₂O₃ nanorods (Samples S180 and S200) at the expense of the α-Fe₂O₃ nanoparticles. In particular, it is evident that the valve-assisted autoclave approach, as used here, has revealed additional details on the growth mechanisms of these nanostructures on the localised scale.

4.2 Stages of growth

The development of large single crystalline α-Fe₂O₃ nanorods can be considered as a two stage process: 1) the growth and dissolution of intermediate β-FeOOH nanostructures, alongside precipitation of α-Fe₂O₃ nanoparticles; and 2) the agglomeration and coarsening of α-Fe₂O₃ primary nanoparticles into α-Fe₂O₃ nanorods.

β-FeOOH has been described as being slightly metastable and therefore kinematically accessible when precipitated from aqueous solution.⁴² However, due to the very small (< 50 nm) β-FeOOH nanoparticle and nanorod sizes, with positive surface energies by implication, the easily hydrated particle surfaces reduce their effective surface enthalpies by up to 0.1 J/m².⁴³ Consequently, β-FeOOH nanoparticles become partially thermodynamically stabilized. The Ostwald rule of stages concerns the competition between kinetics, irreversible thermodynamics and equilibrium thermodynamics.⁴⁴ A decrease in total surface energy is considered to be the driving force for the crystal growth kinetics of β-FeOOH and α-Fe₂O₃. As the more thermodynamically stable α-Fe₂O₃ primary particles nucleate and grow, driven by the enthalpy for bond formation at increasing hydrothermal temperatures, the equilibrium Fe³⁺ concentration at the surface of α-Fe₂O₃ becomes lower than that of β-FeOOH. The resulting concentration gradient leads to Fe³⁺ ions flowing from β-FeOOH to the newly formed α-Fe₂O₃ nanoparticles, prompting β-FeOOH dissolution to restore the equilibrium Fe³⁺ concentration at the β-FeOOH surface. An illustration of β-FeOOH dissolution and α-Fe₂O₃ crystal growth based on the Ostwald rule of stages is shown in Fig. 8a.⁴⁴

Fig. 8 Schematic representation showing: (a) dissolution of intermediate β-FeOOH nanostructures (grey shading) alongside precipitation of α-Fe₂O₃ nanoparticles (lined shading), based on the Ostwald rule of stages; and (b) agglomeration, rotation (arrowed) and coarsening of α-Fe₂O₃ primary particles into an α-Fe₂O₃ nanorod, based on the oriented attachment mechanism.

Further, it is considered that phosphate absorption on the α-Fe₂O₃ nanoparticle surfaces acts to stabilise the < 10 nm particle sizes,^45,46 being distinct from the larger (< 150 nm) isotropic α-Fe₂O₃ nanoparticles which form under surfactant-free conditions.³⁶ These primary α-Fe₂O₃ nanoparticles may be considered as ‘building blocks’ that collide and coalesce in an oriented fashion to form the single crystalline lenticular α-Fe₂O₃ nanorods. It is suggested that preferential phosphate absorption on specific α-Fe₂O₃ crystal faces is consistent with acting to promote the oriented attachment (OA) mechanism^47,48 (Fig. 8b). If the α-Fe₂O₃ nanoparticles are not initially in compatible orientations at the point of coalescence (Fig. 5d), they are still free to rotate to achieve structural coherence at a common interface (Fig. 5a and 5c). It is considered that this grain-rotation-induced grain coalescence (GRIGC) mechanism^49,50 acts to minimize the area of high energy interfaces, allowing low-energy configurations to become established, eliminating misoriented grain boundaries and forming coherent grain-grain boundaries in accordance with the model proposed by Zhang et al.⁴⁷ This OA mechanism appears to be equally applicable to the initial development of α-Fe₂O₃ nanorods (Fig. 5a), as to the attachment of α-Fe₂O₃ primary nanoparticles to well-defined crystalline α-Fe₂O₃ nanorods, creating the filamentary features observed at the nanorod tips (Fig. 5c).

These two distinct stages of crystal growth will be now discussed in more detail, with emphasis on the pivotal role of the phosphate surfactant on the development of the lenticular shape of the α-Fe₂O₃ nanorods.

4.3 Growth and dissolution of β-FeOOH

The evidence from Samples S100 and S120 is consistent with the initial formation of β-FeOOH nanoparticles (∼ 15 nm in diameter, Fig. 3a) followed by the development of β-FeOOH nanorods (∼ 50 nm long, 15 nm wide, Fig. 3b) with tetragonal crystal structure (Fig. 9). The large channels associated with the principal growth c-axis (Fig. 9c) are considered also to be energetically favourable for both growth and dissolution. It is suggested that chlorine and phosphate species occupy these channels, becoming incorporated within the β-FeOOH lattice during HS.^31,51 It is likely that subsequent dissolution of β-FeOOH nanorods occurs along the c-axis, with consequent release of any incorporated chemical species back into solution. The marked reduction in the number of β-FeOOH nanorods and degradation into smaller β-FeOOH nanoparticles with increasing temperature (Sample S160), as evidenced by associated SAED patterns (Fig. 2d), is consistent with this process of β-FeOOH dissolution alongside the development of α-Fe₂O₃ nanoparticles. From a processing point of view, the control of phosphate concentration (combined with temperature and time) provides a mechanism to mediate the supply of Fe³⁺ ions used for the growth of α-Fe₂O₃.

Fig. 9 Schematic representation of the tetragonal β-FeOOH crystal structure showing: projections along the (a) <010> and (b) <110> zone axes; and (c) scaffold-like face normal to the c-axis revealing large channels. Small (red) and large (blue) atoms denote iron and oxygen, respectively. Terminal hydrogen atoms are not shown.

4.4 Form of the phosphate surfactant

Subtle differences in the Fe 2p data for β-FeOOH and α-Fe₂O₃ (Fig. 6a) demonstrate that α-Fe₂O₃ (nanoparticles and some nanorods) had become the dominant phase by 160 °C (Sample S160), showing a similar XPS signature to that exhibited by fully formed lenticular nanorods synthesised at 200 °C (Sample S200). Further, noticeable P 2p peaks demonstrated the association of phosphorus, believed to be in the form of a phosphate complex, with both the β-FeOOH and α-Fe₂O₃ surfaces at all synthesis temperatures (Fig. 6b). In a similar fashion to α-Fe₂O₃, it is considered that these phosphate species act to restrict the size of the β-FeOOH nanostructures (< 50 nm), as compared with β-FeOOH nanostructures (< 60 nm) synthesised in a phosphate-free environment.³⁶ Indeed, phosphate complexes on iron hydroxide are known to depend on pH,^52–54 with a general increase in adsorption being associated with decreasing pH. The HS reaction product solution used here exhibited an initial pH < 2, whilst the zero point charge (zpc) of β-FeOOH is ∼ 8. Hence, it is evident that the high phosphate absorption associated with electrostatic attraction acts to suppress both β-FeOOH growth and dissolution.⁵⁵

Further, the FT-IR evidence presented here is consistent with the absorption of phosphate species on the iron atoms at the α-Fe₂O₃ surfaces, in the form of either mono or bi-dentate complexes. Hence, the form and the role of the phosphate ions attached to α-Fe₂O₃ needs to be considered in more detail. Assignment of the peaks in the FT-IR spectra of Fig. 7 is made with reference to established literature.^56–62 For example, Arai et al.⁵⁶ applied FT-IR spectroscopy to investigate surface phosphate complexes on FeOOH surfaces at low pH values, whilst Rose et al.⁵⁷ and Wilhelmy et al.⁵⁸ both suggested mono-dentate phosphate complexation with ferric ions at pH < 2 when using ferric chloride precursors under specific conditions. In this context, the single broad FT-IR peak (Fig. 7b) for Sample S100 at ∼ 1050 cm⁻¹ can be deconvoluted into peaks at 997, 1056 and 1111 cm⁻¹ and these ν₃ bands, consistent with C_2ν symmetry or lower,⁵³ are in good agreement with the three ν₃ band (see *Footnote) splitting signatures of mono or bi-protonated mono-dentate mononuclear phosphate complexes (FeHPO₄²⁺ or FeH₂PO₄²⁺, both with C₁ symmetry).⁵⁸ The additional peak at ∼ 850 cm⁻¹ is characteristic of the β-FeOOH phase.⁵⁹

In particular, the FT-IR spectra from Samples S160 and S200 (Fig. 7) provide valuable clues as to the nature of the phosphate complexes on the surfaces of both the α-Fe₂O₃ nanoparticles and nanorods. Elzinga et al.⁶⁰ reported on IR spectra of phosphate complexes bonding in a bi-dentate (bridging) fashion with hematite surfaces at low pH values. Accordingly, it is considered that the hematite rich reaction products examined here (Samples S160 to S200, pH ∼1) exhibited similar phosphate bi-dentate surface complexes. Deconvolution of the IR spectrum of Sample S160 revealed strong peaks at 898 934, 970, 1004, 1036, 1071 and 1149 cm⁻¹, (Fig. 7d) which are all consistent with ν₃ bands, with the exception of one ν₁ band at 898 cm⁻¹ (see *Footnote). The three ν₃ bands at 934, 1036 and 1071 cm⁻¹ are assigned to the presence of H₂PO₄²⁻ surface species with C_2ν symmetry or lower.⁵⁸ According to Borgnino et al.,⁶¹ this group of shifted H₂PO₄²⁻ vibrations are indicative of ‘inner-sphere surface complexes,’ attributable to the initial formation of the bi-dentate complex (FeO)₂PO₂.⁶² It is reasonable to assume this complex becomes protonated ((FeO)₂P(OH)₂ with C_2ν symmetry,⁶⁰ at the low pH conditions associated with the HS system examined here. However, the three ν₃ bands at 970, 1004 and 1136 cm⁻¹, also indicative of C_2ν symmetry or lower, are attributable either to a mono-protonated bi-dentate complex (FeO)₂(OH)PO or protonated mono-dentate surface complex (FeO)(OH)₂PO (both with C₁ symmetry).^60–62

4.5 Role of the phosphate surfactant

The coordination of phosphate species on the surface of α-Fe₂O₃ (nanoparticles and nanorods) may be considered in terms of the orientation of the crystal lattice. It has been suggested that the surfactant is adsorbed preferentially on faces parallel to the c-axis of α-Fe₂O₃, in view of the better matching of the O–O inter-atomic distance of PO₄³⁻ anions (2.50Å) with the Fe-Fe spacing parallel rather than perpendicular to this axis.⁶³ Modelling using Carine software⁶⁴ showed the smallest Fe-Fe distances within the α-Fe₂O₃ lattice oriented parallel and normal to the c-axis to be 2.88Å and 5.02Å, respectively (Fig. 10). Hence, it is considered that the crystal structure of α-Fe₂O₃ surface parallel to the c-axis is favourable for the formation of bridging bi-dentate phosphate complexes, whilst such complex formation is not favourable on the c-plane. By implication, it is considered that mono-dentate mononuclear phosphate complex absorption is associated more closely with the c-plane. On this basis, configurations for the most strongly absorbed phosphate complexes on faces normal and parallel to the c-axis (without protonation) are proposed in Fig. 10a and 10b, respectively. The association of the mono-dentate phosphate complex with the c-plane implies weaker absorption, as compared with the bi-dentate phosphate complex associated with planes parallel to the c-axis. This disparity is considered critical for mediating the attachment of the primary α-Fe₂O₃ nanoparticles, and through the filamentary feature growth (Fig. 5c), governs the lenticular shape of the developing α-Fe₂O₃ nanorods. The inference is that it is easier for such mono-dentate surfactant complexes to be displaced during α-Fe₂O₃ growth, as compared with bidentate complexes, consistent with the α-Fe₂O₃ crystal c-faces being favourable surfaces for oriented attachment.

Fig. 10 Schematic diagram of the tetragonal α-Fe₂O₃ crystal lattice showing (a) monodentate phosphate adsorption on the c-plane; and (b) bidentate phosphate adsorption on a faces parallel to the c-axis (Red (small), blue (large) and green (smallest) spheres correspond to iron, oxygen and phosphorus, respectively).

4.6 Growth mechanism

The schematic diagram of Fig. 11 summarises the results of this in situ ‘snapshot’ approach, as used to investigate the hydrothermal growth of α-Fe₂O₃ nanorods as a function of temperature. During Step 1 (100 °C), hydrothermal synthesis promotes the initial precipitation of kinetically accessible β-FeOOH nanoparticles and nanorods and a few α-Fe₂O₃ nanoparticles. Step 2 (120 °C) illustrates the further nucleation of α-Fe₂O₃ nanoparticles at increased temperature. During Step 3 (140 °C), β-FeOOH dissolution begins to occur in response to the Fe³⁺ concentration gradient established with the additional nucleation of the more thermodynamically stable α-Fe₂O₃ nanoparticles. This further increase in temperature also provides the driving force for the suggested oriented attachment of α-Fe₂O₃ nanoparticles, mediated by the stronger adsorption of bi-dentate (bridging) phosphate inner sphere complexes to faces parallel to the α-Fe₂O₃ c-axis. Step 4 (160 °C) illustrates the degradation of β-FeOOH nanorods to a size similar to that of the α-Fe₂O₃ nanoparticles, through dissolution of loosely packed c-planes. Thereafter, the α-Fe₂O₃ nanorods grow and coarsen further during Steps 4 to 6 (160–200 °C) through the oriented attachment mechanism, until the supply of the primary α-Fe₂O₃ nanoparticles is exhausted. The highest hydrothermal temperature of 200 °C, after a processing time of 80 min in this instance, results in the production of large, lenticular, single crystalline α-Fe₂O₃ nanorods.

Fig. 11 Schematic diagram summarising the HS growth mechanism for α-Fe₂O₃ nanorods, as a function of increasing temperature: (1) precipitation of β-FeOOH nanoparticles and nanorods (grey), and α-Fe₂O₃ nanoparticles (brown); (2) nucleation of additional α-Fe₂O₃ nanoparticles; (3) dissolution of β-FeOOH nanorods alongside oriented α-Fe₂O₃ nanoparticle attachment; (4) dissolution of β-FeOOH nanorods into nanoparticles and growth of α-Fe₂O₃ nanorods; (5) growth and coarsening of α-Fe₂O₃ nanorods; and (6) large, single crystalline α-Fe₂O₃ nanorods.

5. Conclusions

The application of a novel, valve-assisted, pressure vessel has allowed ‘snapshots’ of the reaction products formed during hydrothermal synthesis to be obtained as a function of reaction temperature, providing near in situ evidence for the growth of α-Fe₂O₃ nanorods. Precipitation and dissolution of the intermediate phase of β-FeOOH (considered favourable along the c-axis) was observed with increasing temperature. The release of Fe³⁺ ions back into solution supplied the nucleation and growth of primary α-Fe₂O₃ nanoparticles which subsequently coalescence, with increasing temperature, into larger, lenticular α-Fe₂O₃ nanorods. The difference between the strong phosphate absorption to faces parallel to the α-Fe₂O₃ c-axis, through a bidentate (bridging) surface complex, as distinct from the weaker monodenate phosphate absorption associated with the α-Fe₂O₃ c-plane, is considered to be critical in mediating the shape of the developing, single crystalline, lenticular α-Fe₂O₃ nanorods.

Acknowledgements

With thanks the EPSRC for funding (grant ref: EP/P50354X/1). Thanks, also, to Emily Smith from the School of Chemistry, and to Adil Rehmen from the Department of Mechanical, Materials and Manufacturing Engineering, Faculty of Engineering, The University of Nottingham, for providing XPS data and reaction solution temperature measurements, respectively.

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Footnote

† Present address: College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, EX4 4QF. E-mail: yanqiu.zhu@exeter.ac.uk

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