Acetate-induced controlled-synthesis of hematite polyhedra enclosed by high-activity facets for enhanced photocatalytic performance

Abstract

Uniform hematite polyhedra were fabricated by a facile acetate-induced solvothermal method. Based on a surface activity order of {101} > {012} > {001}, α-Fe₂O₃ truncated bipyramids dominantly enclosed by {101} facets showed the best performance regarding the photodegradation of RhB with a degradation rate constant of 0.01051 min⁻¹.

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Under the background of sustainable development, environmental issues have been the headlines in current society. Among the research studies on pollution problems, wastewater treatment account for quite a considerable proportion, e.g. adsorption and photocatalytic degradation.¹ Therein, photocatalytic processes are a promising route that employs photocatalysts to transform inexhaustible solar energy into chemical energy towards decaying organic pollutants in wastewater.² Since surface structure significantly influence the photocatalytic behavior of a catalyst, it's significant to rationally engineer surface structures, especially regulating the morphologies to expose more high-activity facets outwards.³ For example, it has been reported that for silver phosphate (Ag₃PO₄) crystals, exposed {110} facets exhibit better photocatalytic capacity than {001} facets for the degradation of methyl orange (MO) and rhodamine B (RhB) dyes⁴ while {002} facets for zinc oxide (ZnO),⁵ {001} facets for titania (TiO₂)⁶ and {111} facets for cuprous oxide (Cu₂O) show high activity.⁷ Thus, exposing more high-activity facets is regarded as an efficient way to enhance the photocatalytic performance.

Among photocatalysts, hematite (α-Fe₂O₃), has drawn intensive attention for its advantages of excellent photocatalytic performance and abundant resource in nature.^8a α-Fe₂O₃, an ordinary semiconductor material with a favorable bandgap of 1.9–2.3 eV,⁹ is thermodynamically stable in all the iron oxides, even in aqueous acid solution with a relative low pH value of 3 or under high-temperature conditions. Therefore, α-Fe₂O₃ has been widely used in ions batteries, water splitting reactions, sensing and catalytic fields, etc.¹⁰ For α-Fe₂O₃ crystals, surface structure significantly influences the physicochemical properties, especially photocatalytic activity. It has been demonstrated that the photocatalytic performances of α-Fe₂O₃ catalysts are notably enhanced by the exposed high-activity facets. For example, Wang and co-workers have confirmed that α-Fe₂O₃ polyhedra with {101} facets exposed exhibit much higher catalytic activity than those with {001} facets.¹¹ Zhou et al. have reported that α-Fe₂O₃ crystals enclosed by {012} facets outperform those enclosed by {001} facets in photodegradation of organic dyes.¹² In addition, Ding and co-workers have declared that α-Fe₂O₃ crystals surrounded by {21̄5} facets possess higher activity than pseudocubes of {102} facets in photogradation process.¹³ Among current synthetic strategy to promote the exposure of high-activity facets, toxic additives (e.g. formamide) or complicated reaction systems which may have negative effects on the environment are generally adopted to control the preferred growth direction of α-Fe₂O₃ crystals.¹⁴ Therefore, it's considerably significant to develop a nontoxic and facile synthetic strategy for α-Fe₂O₃ crystals to expose high-activity facets.

Herein, we demonstrate a novel solvothermal route by employing nontoxic univalent acetates as inducer and water–ethyl alcohol (H₂O–EtOH) mixture as reaction system to promote the exposure of high-activity facets, e.g., {101} or {012}. Adjusting the acetate species in synthetic process, α-Fe₂O₃ polyhedra of uniform morphologies with desirable facets exposed were successfully prepared. Benefited from the exposed high-activity {101} facets, α-Fe₂O₃ truncated bipyramids exhibited the best catalytic performance in the photodegradation of RhB, resulting in the largest degradation rate constant.

The phase of the product obtained via a solvothermal process with potassium acetate (KAc) added was evaluated by X-ray diffraction (XRD) (black pattern shown in Fig. S1, ESI†). All of the diffraction peaks are readily indexed to rhombohedral α-Fe₂O₃ structure (JCPDS 24-0072, space group: R3̄c (no. 167)) with lattice constants of a = 0.5036 nm and c = 1.3749 nm. No other peaks are detected, suggesting that the α-Fe₂O₃ product of high purity is successfully synthesized. The sharp diffraction peaks also show that the obtained α-Fe₂O₃ product is well-crystallized. To investigate morphology information, the α-Fe₂O₃ product was characterized by scanning electron microscopy (SEM). Fig. 1A and B depict the typical SEM images at different magnifications of the product obtained in the presence of KAc. From SEM result, it can be clearly seen that the particles have a polyhedral surface with an average size of ∼250 nm. The higher magnified SEM image in the top of Fig. 1C clearly displays the particle has truncated bipyramids morphology with a 6-fold axis and is enclosed by 12 oblique side facets and 2 flat top facets, which can be simulated as a geometric model (shown in the bottom of Fig. 1C). Fig. 1D shows a typical TEM image of the obtained product. Each particle exhibits a symmetrical structure. The inset in Fig. 1D is a select area electron diffraction (SAED) pattern collected at the marked circular region in Fig. 1D. The sharp regular spots reveal the single-crystalline nature of the α-Fe₂O₃ polyhedron with high crystallinity. The spots are indexed to {110}, {003} and {113} facets under the incident electron beam along [11̄0] direction. It is noted that the intersection angle between {110} and {003} facets is precisely 90°, agreeing well with their crystallographic relation. The high-resolution TEM (HRTEM) image is presented in Fig. 1E. A two-dimension lattice fringes can be clearly resolved, indicating the particle is homogeneous structurally with an interplanar spacing of about 0.416 nm and 0.270 nm, which can correspond to the (101̄) and (104) lattice planes, respectively, revealing that the lattice fringes were derived from the projection of α-Fe₂O₃ along the [010] direction (as depicted in Fig. 1F). Based on the HRTEM result, it's concluded that the 12 equivalent side facets can be attributed to {101} facets while the top and bottom facets are {001}.¹⁵

Fig. 1 (A, B and the upper part of C) SEM images of the as-prepared α-Fe₂O₃ truncated bipyramids at different magnification; (the bottom of C) a model of a single particle; (D) TEM image and (E) HRTEM image of the as-prepared α-Fe₂O₃ product, the inset in (D) is a SAED pattern; (F) crystal structure of α-Fe₂O₃ product along [010] direction. Small red balls are oxygen atoms while large brown spheres stand for iron atoms.

In current synthetic approach, KAc, acting as an alkali source donator, plays a crucial role to the formation of α-Fe₂O₃ truncated bipyramids on basis of the reaction process: Ac⁻ + H₂O ↔ HAc + OH⁻. Under the high temperature condition, Ac⁻ ions contact with water molecule and hydrolysis, releasing OH⁻ anions slowly. The released OH⁻ anions can combine with iron ions (Fe³⁺) derived from ferric chloride (FeCl₃·6H₂O) raw material, thus, unstable Fe(OH)₃ is generated. Under elevated temperature condition, Fe(OH)₃ decomposes to produce the final product α-Fe₂O₃, accompanied with the formation of water. In the reaction system, the low content of water in H₂O–EtOH mixture can limit the hydrolysis rate of Ac⁻ anions, by which the release rate of OH⁻ anions is controlled. Meanwhile, Ac⁻ anions have strong chelating effect with transition metal ions (e.g. Fe³⁺ ions) inherently, therefore, the excess Ac⁻ anions could coordinate with α-Fe₂O₃ crystals on specific planes and promote the growth along defined direction.¹⁶ On the other hand, it has been reported that the presence of divalent transition metal ions (e.g. Zn²⁺ and Cu²⁺ ions¹⁷) could tailor the growth direction as well as the shape of α-Fe₂O₃ crystals. Thus, it's reasonable to propose that due to the different adsorption properties to specific surface of α-Fe₂O₃ crystals, various univalent alkali metal ions (e.g. K⁺ ions) might also affect the formation of α-Fe₂O₃ polyhedra, leading to different featured facets. To verify this conjecture, experiments were conducted by using ammonium acetate (NH₄Ac) and sodium acetate (NaAc) to replace initial KAc as additives.

The crystal structure of the products, obtained by adding NH₄Ac and NaAc, respectively, were confirmed by the corresponding XRD patterns shown in Fig. S1.† Both of the two patterns match well with that of α-Fe₂O₃ truncated bipyramids and no peaks of other impurities are observed, suggesting the products are of high purity. Fig. 2A is the SEM image of α-Fe₂O₃ prepared by adding NH₄Ac, which shows that the α-Fe₂O₃ particles are well detached and have a fine uniformity with an average of ∼90 nm. The inset of Fig. 2A is a magnified SEM image, showing the powder is of pseudocube morphology, and its corresponding geometric model is depicted as the inset of Fig. 2B. Fig. 2B shows a TEM image of α-Fe₂O₃ pseudocubes. The particles are well-dispersed with an even size which is identical to the SEM result. The HRTEM measurement is also conducted on the transmission electron microscope to confirm the microstructure (depicted in Fig. 2C). The HRTEM image obviously shows an interlaced two-dimension lattice fringes with a dihedral angle of about 86° and an interplanar spacing of 0.368 nm, indexing to be (012) and (102̄) lattice planes, respectively. The inset in Fig. 2C displays a corresponding SAED image. Likewise, the sharp spots matrix clearly shows the intrinsic single-crystal character and can conform to {012}, {102̄} and {110} facets under the incident electron beam along the [22̄1] direction. According to HRTEM and SAED results, the other surface is deduced to be (11̄2). Thus, the exposed facets of the ideal α-Fe₂O₃ pseudocubes could be identical to {012}, {102̄} and {11̄2}. Interestingly, when KAc is substituted by NaAc, α-Fe₂O₃ with new morphology were also obtained. Fig. 2D and E are the SEM image and TEM image of α-Fe₂O₃ prepared by adding NaAc as inducer, respectively, in which hexagonal nanoplates with an average size of ∼150 nm in size are depicted. Based on the morphology results, a hexagonal nanoplate-like geometric model is built (shown as the inset in Fig. 2E). To further investigate the crystal structure, HRTEM is conducted (depicted in Fig. 2F). Similar with the HRTEM image in Fig. 2C, Fig. 2F also shows an interlaced two-dimension lattice fringes with a dihedral angle of about 60° and both of two groups of facets have an interplanar spacing of 0.252 nm, corresponding to (2̄10) and (110) facets, respectively. The inset in Fig. 2F is an identical SAED image, in which the spots suggest its single-crystal nature and can be signed to {2̄10}, (1̄20) and {110} facets and/or their equivalent facets under the incident electron beam along the [001] direction. Thus, the ideal model of hexagonal nanoplates is dominantly enclosed by (001) facets. From the above, adding different univalent acetates (containing K⁺, NH₄⁺ or Na⁺ ions) in solvothermal process, α-Fe₂O₃ truncated bipyramids, pseudocubes and nanoplates with different activity facets exposed were successfully obtained separately. Generally, the formation of polyhedral architectures is caused by different growth rate along particular directions. Specifically, the growth rate of a defined direction is limited by surface adsorbed ions. As the hydrated radius of cations employed in this work obeys the order of Na⁺ < NH₄⁺ < K⁺,¹⁸ the electrostatic adsorption ability on oxide surfaces in crystal nucleation and growth process may follow the opposite sequence. Induced by electrostatic interaction, univalent cations of smaller radius tend to adsorb on the surface with more oxygen atoms exposed. Thus, apart from the presence of Ac⁻ anions, the addition of univalent cations (K⁺, NH₄⁺, Na⁺) seems to affect and regulate the growth rate of different α-Fe₂O₃ facets with variant density of surface atoms, as will elucidated later.

Fig. 2 (A) SEM, (B) TEM and (C) HRTEM images of α-Fe₂O₃ pseudocubes, the insets in (A), (B) and (C) are a magnified SEM image, a geometric model and a SAED pattern, respectively; (D) SEM, (E) TEM and (F) HRTEM images of α-Fe₂O₃ nanoplates, the insets in (D), (E) and (F) are a magnified SEM image, a geometric model and a SAED pattern, respectively.

As is known, α-Fe₂O₃ polyhedra with specific index facets exposed tend to have considerable performance in catalytic field. And the photocatalytic activity of α-Fe₂O₃ materials could be assessed by the photodegradation of RhB in the presence of hydrogen peroxide (H₂O₂) under UV-vis irradiation. The intensity of absorption peak around 550 nm was adopted to represent the concentration of aqueous RhB solution, thus the decrease of intensity as the time prolonged was used to character the photocatalytic performance of α-Fe₂O₃ materials. Fig. 3A shows time-dependent UV-vis absorption spectra of RhB solution during photodegradation process in the presence of α-Fe₂O₃ truncated bipyramids under UV-vis irradiation, from which it can be observed visually that the high-concentration RhB solution is almost degraded completely after 30 min. The practical degradation result is shown intuitively as the inset of Fig. 3A. Obvious color change of the solution was observed and the last group seems to be colorless. The variation of photodegradation degree (signed as c/c₀, wherein c and c₀ are the concentration at a random reaction time and the initial concentration, respectively) of RhB solution with the irradiation time in the presence of these three classes of catalysts, α-Fe₂O₃ truncated bipyramids, pseudocubes and nanoplates, is compared in Fig. 3B. The photodegradation degree follows the order that α-Fe₂O₃ truncated bipyramids exhibit dramatically superior photodegradation activity on RhB solution to α-Fe₂O₃ pseudocubes, which manifest higher photocatalytic performance than α-Fe₂O₃ nanoplates. In the whole degradation process, α-Fe₂O₃ truncated bipyramids exhibits the best result continuously in comparison. Moreover, it can be seen that irradiated by Xe light for 30 min, the degradation efficiency of RhB is 98.08%, 90.30% and 81.14% for α-Fe₂O₃ truncated bipyramids, pseudocubes and nanoplates, respectively (while 22.63% for the blank test). In addition, Fig. 3C depicts that after continuous irradiation for 15 min, the degradation degree of RhB achieves to be 80.04% for α-Fe₂O₃ truncated bipyramids, which is notably higher than that for α-Fe₂O₃ pseudocubes (52.46%) and nanoplates (43.14%). Notably, the degradation value of 80.04% for α-Fe₂O₃ truncated bipyramids is obviously higher than 60% for α-Fe₂O₃/graphene composite with a weight ratio of 1% for GO.¹⁹

Fig. 3 (A) Time-dependent UV-vis absorption spectra in the presence of α-Fe₂O₃ truncated bipyramids; (B) comparison of photodegradation of RhB by α-Fe₂O₃ enclosed by specific facets in 30 min; (C) photocatalytic performance of α-Fe₂O₃ polyhedra in 15 min; (D) kinetic curves of the degradation of the photodegradation of RhB.

Previous reports show that the degradation of RhB molecule can be regarded as a pseudo-first order reaction and the photocatalytic reaction kinetics follow the formula: ln(c₀/c) = kt, where k is the degradation rate constant while t is the reaction time and c₀ and c refer to the same with above statement.¹⁹ To further compare the distinctions of photocatalytic degradation activity of the three catalysts, rate constant k is adopted to evaluate the reaction rate of a photocatalytic process. The variations of the value of ln(c₀/c) as a function of reaction time are shown in Fig. 3D. On basis of the calculated kinetic curves, α-Fe₂O₃ truncated bipyramids have the largest rate constant, 0.1051 min⁻¹, which is about twice and three times as high as that of α-Fe₂O₃ pseudocubes (0.0484 min⁻¹) and nanoplates (0.0342 min⁻¹), separately. Generally, the photocatalytic activity of α-Fe₂O₃ crystals is mainly determined by surface structure and surface area, etc. In current degradation comparisons, it's obviously adverse for α-Fe₂O₃ truncated bipyramids in surface area due to its larger size than that of α-Fe₂O₃ pseudocubes and nanoplates of nanoscale. This abnormal degradation result may be ascribed probably to the surface structure of exposed facets of the three catalysts.

It's generally accepted that the surface structure, including surface electronic and atomic structure, may influence photocatalytic reactivity. UV-vis diffuse reflectance spectra (DRS) were adopted to investigate the surface electronic structure of the catalysts. After Kubelka–Munk transformation of the reflection as a function of wavelength, the spectra suggest that all the α-Fe₂O₃ catalysts share a comparative absorption edge at the range of 550–600 nm (shown in Fig. S2, ESI†). The inset is the calculation diagram of the bandgaps, which is determined to be 2.10, 2.16 and 2.17 eV for α-Fe₂O₃ truncated bipyramids, pseudocubes and nanoplates, respectively. The as-prepared α-Fe₂O₃ catalysts show comparable surface electronic structure, meanwhile, the visual differences in photocatalytic performance could be rationalized by the intrinsic differences in atomic structures of exposed facets.

Table S1 (shown in ESI†) lists the dominant exposed facets of the three α-Fe₂O₃ catalysts. Previous studies have shown that nanoplates mainly enclosed by {001} facets were terminated by either a single oxygen or iron layer (shown as Fig. 4A and B). In aqueous solution, the crystallographically distinct terminations could coexist and affect the reactivity. The density of surface iron atoms of {001} facets is 0 and 4.6 atoms per square nanometer for an oxygen and iron terminated layer respectively, less than 7.3 atoms per square nanometer of {012} facets. As a result, {001} facets exhibit lower photocatalytic activity than {102} ones.²⁰ For pseudocubes, the exposed {012}, {11̄2} and {102̄} facets are equivalent in crystallographic arrangement and atomic structure. Thus, the photocatalytic results of α-Fe₂O₃ nanoplates and pseudocubes are identical with the atomic structures of the exposed facets. In addition, for the mixed oxygen and iron terminations of exposed {101} facets, the density of surface iron atoms is 8.3 atoms per square nanometer, higher than that of {102} facets. Furthermore, unlike the iron atoms in bulk, half of iron atoms of {101} facets are coordinated with five oxygen atoms and the rest are tetracoordinated (depicted as Fig. 4D) while each iron atom of {012} facets is pentacoordinated with oxygen atoms (shown in Fig. 4C). More unsaturated coordination may generate nonlinear higher electrostatic adsorption capacity for RhB. Meanwhile, the ridge-and-valley structure of exposed {101} surface is helpful for the adherence of RhB molecule on coordinatively unsaturated iron atoms. Since the degradation of RhB is greatly influenced by iron cations of +3 state, which can lead to the generation of hydroxy radicals (˙OH), to decompose dye molecules. Hence, the photocatalytic activity of exposed facets follows the order: {101} > {012} > {001}, which is identical to surface energies order of quantitative analysis previously,^8b,21 while the photocatalytic performance of α-Fe₂O₃ polyhedra obeys the sequence of truncated bipyramids > pseudocubes > nanoplates. Compared with previous reports on photocatalysts for Fenton reaction, current finding claims that surface engineering rationally is significant in photocatalytic field.

Fig. 4 Side views of exposed facets terminations: {001} with (A) oxygen and (B) iron atoms terminations; (C) {012} and (D) {101}. The dark arrows refer to the unsaturated iron atoms coordinated with five oxygen atoms and the light arrow is tetracoordinated iron atoms.

In summary, a solvothermal synthetic strategy was demonstrated for the controllable preparation of uniform α-Fe₂O₃ truncated bipyramids, pseudocubes and nanoplates by introducing KAc, NH₄Ac and NaAc as inducer, respectively. Benefited from surface activity of {101} > {012} > {001} facets, α-Fe₂O₃ truncated bipyramids performed the best on the photodegradation of RhB under UV-vis light irradiation and had the largest degradation rate constant. This work shows great potential to tailor the exposed facets of metal oxides semiconductors by adding inorganic salts as inducer for surface engineering to enhance photocatalytic performance.

Acknowledgements

The authors acknowledge the financial support by National Natural Science Foundation of China (51372279), Hunan Provincial Natural Science Foundation of China (13JJ1005). X. L. acknowledges support from Shenghua Scholar Program of Central South University. R. M. acknowledges support from JSPS KAKENNHI (24310095).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07809e

‡ These authors contributed equally to this work.

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