Synthesis of ordered mesoporous Fe3O4 and gamma-Fe2O3 with crystalline walls using post-template reduction/oxidation

Ordered mesoporous Fe(3)O(4) with crystalline walls (inverse spinel structure) has been synthesized for the first time, representing to the best of our knowledge, the first synthesis of a reduced mesoporous iron oxide. Synthesis was achieved by reducing ordered mesoporous alpha-Fe(2)O(3) (corundum structure) to Fe(3)O(4) spinel then to gamma-Fe(2)O(3) by oxidation, while preserving the ordered mesostructure and crystalline walls throughout. Such solid/solid transformations demonstrate the stability of the mesostructure to structural phase transitions from the hexagonal close packed oxide subarray of alpha-Fe(2)O(3) (corundum structure) to the cubic close packed subarray of Fe(3)O(4) spinel and gamma-Fe(2)O(3). Preliminary magnetic measurements reveal that the spins in both Fe(3)O(4) and gamma-Fe(2)O(3) are frozen at 295 K, despite the wall thickness (7 nm) being less than the lower limit for such freezing in corresponding nanoparticles (>8 nm).     

Preparation and adsorption properties of monodisperse chitosan-bound Fe3O4 magnetic nanoparticles for removal of Cu(II) ions

Monodisperse chitosan-bound Fe(3)O(4) nanoparticles were developed as a novel magnetic nano-adsorbent for the removal of heavy metal ions. Chitosan was first carboxymethylated and then covalently bound on the surface of Fe(3)O(4) nanoparticles via carbodiimide activation. Transmission electron microscopy micrographs showed that the chitosan-bound Fe(3)O(4) nanoparticles were monodisperse and had a mean diameter of 13.5 nm. X-ray diffraction patterns indicated that the magnetic nanoparticles were pure Fe(3)O(4) with a spinel structure, and the binding of chitosan did not result in a phase change. The binding of chitosan was also demonstrated by the measurement of zeta potential, and the weight percentage of chitosan bound to Fe(3)O(4) nanoparticles was estimated to be about 4.92 wt%. The chitosan-bound Fe(3)O(4) nanoparticles were shown to be quite efficient for the removal of Cu(II) ions at pH>2. In particular, the adsorption rate was so fast that the equilibrium was achieved within 1 min due to the absence of internal diffusion resistance. The adsorption data obeyed the Langmuir equation with a maximum adsorption capacity of 21.5 mg g(-1) and a Langmuir adsorption equilibrium constant of 0.0165 L mg(-1). The pH and temperature effects revealed that the adsorption capacity increased significantly with increasing pH at pH 2-5, and the adsorption process was exothermic in nature with an enthalpy change of -6.14 kJ mol(-1) at 300-330 K.     

In situ observation of gamma-Fe2O3 nanoparticle adsorption under different monolayers at the air/water interface

We studied the adsorption of gamma-Fe 2O 3 nanoparticles from an aqueous solution under different charged Langmuir monolayers (stearic acid, stearyl alcohol, and stearyl amine). The aqueous subphase was composed of a colloidal suspension of gamma-Fe 2O 3 nanoparticles. The average hydrodynamic diameter of the nanoparticles measured by dynamic light scattering measurements was 16 nm. The observed zeta potential of +40 mV (at pH 4) results in a long-term stability of the colloidal dispersion. The behavior of the different monolayer/nanoparticle composites were studied with surface pressure/area (pi/ A) isotherms. The adsorption of the nanoparticles under the different monolayers induced an expansion of the monolayers. These phenomena depended on the charge of the monolayers. After the Langmuir/Blodgett transfer on glass substrates, the nanoparticle/monolayer composite films were studied by means of UV-vis spectroscopy. The spectra pointed to increasing adsorption of the nanoparticles with increasing electronegativity of the monolayers. On the basis of these results, we studied the in situ adsorption of nanoparticles under the different monolayers by X-ray reflectivity measurements. Electron density profiles of the liquid/gas interfaces were obtained from the X-ray reflectivity data. The results gave clear evidence for the presence of electrostatic interaction between the differently charged monolayers and the positively charged nanoparticles. While the adsorption process was favored by the negatively charged stearic acid monolayer, the positively charged layer of stearyl amine prevented the formation of ultrathin nanoparticle layers.     

Ultrafast energy transfer and strong dynamic non-condon effect on ligand field transitions by coherent phonon in gamma-Fe2O3 nanocrystals

Relaxation dynamics of an optically excited ligand field state and strong modulation of oscillator strengths of ligand field transitions by coherent acoustic phonon in gamma-Fe(2)O(3) nanocrystals were investigated through transient absorption measurements. A near-infrared pump beam prepared the lowest excited ligand field state of Fe(3+) ions preferentially on the tetrahedral coordination site. A time-delayed visible probe beam monitored the dynamics of various ligand field transitions and modification of their oscillator strengths by a coherent lattice motion. Transient absorption data exhibited dynamic features of a few distinct time scales, 100 fs, 1 ps, and 17-100 ps, as well as intense oscillatory features resulting from a coherent acoustic phonon. The initial decay of the induced absorption in 100 fs has been attributed to the exchange interaction-mediated energy transfer from the tetrahedral to octahedral Fe(3+) sites. The dynamics of slower time scales were assigned to the vibrational and electronic relaxations. Excitation of the ligand field state created a coherent acoustic phonon resulting in unusually intense modulation of the transient absorption signal despite its predominantly local nature and relatively small vibronic coupling. Excitation of each Fe(3+) ion in the nanocrystal was estimated to modulate up to 60% of its contribution to the total absorption intensity of the nanocrystal. The intense modulation of the absorption has been attributed to the strongly modulated oscillator strength of the ligand field transitions rather than oscillating Frank-Condon overlap. Dynamic modification of the metal-ligand orbital overlap and exchange interaction between the neighboring metal ions are the main factors responsible for the modulation of the oscillator strength.     

Nanostructured pure gamma-Fe2O3 via forced precipitation in an organic solvent

Pure maghemite, gamma-Fe(2)O(3), was prepared as ultra fine particles in the nanometer-sized range via the forced precipitation method in an organic solvent. The precipitation of iron(III) ions, from iron(III) chloride in 2-propanol led selectively to highly dispersed particles of ferrihydrite, which upon treatment with temperatures higher than 200 degrees C under dynamic vacuum resulted in high-surface-area particles of gamma-Fe(2)O(3). Precipitation in water also led to ferrihydrite, but the final product, after heating at 300 degrees C, contained a mixture of gamma-Fe(2)O(3) and alpha-Fe(2)O(3) (hematite). The precipitation from iron(III) nitrate in water resulted in goethite which was converted to hematite upon heating. On the other hand, the final product in 2-propanol was a mixture of maghemite and hematite. The products were characterized by FTIR, TGA, XRD, and gas sorption analysis. Nitrogen gas adsorption studies for the pure gamma-Fe(2)O(3) samples revealed mesoporous particles with high surface areas in the range of 70-120 m(2) g(-1) after heat treatment at 300 degrees C. The gamma-Fe(2)O(3) particles retained their gamma-phase as well as their mesoporous structure at relatively high temperatures, as high as 400 degrees C.     

Doping gamma-Fe(2)O(3) nanoparticles with Mn(III) suppresses the transition to the alpha-Fe(2)O(3) structure

Here we report on a mixed oxide system, gamma-Fe2O3 nanoparticles doped with Mn(III), where the transition from the cubic to the more stable hexagonal alpha-Fe2O3 structure is suppressed. When amorphous Fe2O3 is heated at 300 degrees C for 3 h, ferrimagnetic gamma-Fe2O3 is observed as the sole product. On the other hand, when the temperature is raised to 500 degrees C, one observes only antiferromagnetic alpha-Fe2O3 as the product. However, upon doping with 8.5 wt % Mn(III), the amorphous nanoparticles crystallized to mainly the gamma-Fe2O3 matrix after heating at 500 degrees C for 3 h, and need to be heated to >650 degrees C for the complete transition to the alpha-Fe2O3 structure to take place.     

介孔α-Fe2O3表面配合反应平衡常数测定

采用十二胺为模板剂、氨水做沉淀剂成功制备了介孔α-Fe2O3,通过粉末X射线衍射(XRD)、透射电镜(TEM)、N2吸附/脱附技术对样品晶相、形貌和比表面积进行了表征.根据介孔α-Fe2O3悬浮液的酸碱滴定数据,使用FITEQL软件,采用双电层恒电容模型计算得出了介孔α-Fe2O3的表面酸碱反应平衡常数.在此基础上研究了Cu2+,Pb2+,Zn2+在介孔氧化铁表面的吸附行为,使用WinSGW软件模拟得出了相应的表面配合反应平衡常数并讨论了其吸附机理.     

Co~(2+)/Dy~(3+)掺杂纳米立方Fe_3O_4的热还原制备及磁靶向滞留性能

采用热还原沉淀法制备了一系列Co~(2+)/Dy~(3+)掺杂的纳米立方MxFe3-xO4磁性颗粒.利用X射线衍射仪、透射电子显微镜和振动样品磁强计研究了不同含量掺杂离子对MxFe3-xO4晶体结构、形貌及磁性的影响.研究发现,掺杂未改变母体的对称性,但母体形貌逐渐从立方体向球体过渡;Co~(2+)和Dy~(3+)的掺杂对于铁氧体磁学性质的影响明显不同,当Co~(2+)实际掺杂量为0.44和Dy~(3+)实际掺杂量为0.05时,MxFe3-xO4立方磁性粒子的饱和磁化强度(Ms)达到最大值,分别为76.65和70.21 A·m2·kg-1.与超顺磁性Fe_3O_4球体相比,高磁性掺杂Fe_3O_4立方体在体外模拟磁流体磁靶向定位实验中显示出较高的滞留率.     

羧甲基化壳聚糖- Fe3O4纳米粒子的制备及对Zn2+的吸附行为

以共沉淀法制备纳米Fe3O4, 通过在颗粒表面接枝羧甲基化壳聚糖(CMC), 制备一种新型磁性纳米吸附剂, 用透射电镜(TEM)、X射线衍射分析(XRD)等对其进行了表征, 并考察了吸附剂对Zn2+的吸附性能. 结果表明, 制备的磁性纳米吸附剂平均粒径18 nm, 粒子中CMC的含量约5%. 该吸附剂对Zn2+吸附速率很快, 在2 min内基本达到平衡, 能有效去除Zn2+. 等温吸附数据符合Langmuir模型, 饱和吸附容量为20.4 mg•g−1, 吸附常数为0.0314 L•mg−1. 热力学计算表明吸附为放热过程, 焓变为−5.68 kJ•mol−1.     

Characterization of electronic transition energies and trigonal distortion of the (FeO6)9- coordination complex in the Al2O3:Fe3+ system: a simple method for transition-metal ions in a trigonal ligand field

A theoretical method for studying the inter-relationships between electronic and molecular structure has been proposed on the basis of the complete energy matrices of electron-electron repulsion, the ligand field, and the spin-orbit coupling for the d5 configuration ion in a trigonal ligand field. As an application, the local distortion structure and temperature dependence of zero-field splitting for Fe3+ ions in the Al2O3:Fe3+ system have been investigated. Our results indicate that the local lattice structure of the (FeO6)(9-) octahedron in the Al2O3:Fe3+ system has an elongated distortion and the value of distortion is associated with the temperature. The elongated distortion may be attributed to the facts that the Fe3+ ion has an obviously larger ionic radius than the Al3+ ion and the Fe3+ ion will push the two oxygen triangles upward and downward, respectively, along the 3-fold axis. By diagonalizing the complete energy matrices, we found that the theoretical results of electronic transition energies and EPR spectra for Fe3+ ions in the Al2O3:Fe3+ system are in good agreement with the experimental findings. Moreover, to understand the detailed physical and chemical properties of the Al2O3, the theoretical values of the zero-field splitting parameters and the corresponding distortion parameters in the range 50 K 相似文献   

Preparation of surface plasmon resonance biosensor based on magnetic core/shell Fe3O4/SiO2 and Fe3O4/Ag/SiO2 nanoparticles

In this paper, surface plasmon resonance biosensors based on magnetic core/shell Fe(3)O(4)/SiO(2) and Fe(3)O(4)/Ag/SiO(2) nanoparticles were developed for immunoassay. With Fe(3)O(4) and Fe(3)O(4)/Ag nanoparticles being used as seeding materials, Fe(3)O(4)/SiO(2) and Fe(3)O(4)/Ag/SiO(2) nanoparticles were formed by hydrolysis of tetraethyl orthosilicate. The aldehyde group functionalized magnetic nanoparticles provide organic functionality for bioconjugation. The products were characterized by scanning electronic microscopy (SEM), transmission electronic microscopy (TEM), FTIR and UV-vis absorption spectrometry. The magnetic nanoparticles possess the unique superparamagnetism property, exceptional optical properties and good compatibilities, and could be used as immobilization matrix for goat anti-rabbit IgG. The magnetic nanoparticles can be easily immobilized on the surface of SPR biosensor chip by a magnetic pillar. The effects of Fe(3)O(4)/SiO(2) and Fe(3)O(4)/Ag/SiO(2) nanoparticles on the sensitivity of SPR biosensors were also investigated. As a result, the SPR biosensors based on Fe(3)O(4)/SiO(2) nanoparticles and Fe(3)O(4)/Ag/SiO(2) nanoparticles exhibit a response for rabbit IgG in the concentration range of 1.25-20.00 μg ml(-1) and 0.30-20.00 μg ml(-1), respectively.     

纳米氧化铁修饰玻碳电极检测痕量镉离子

制备了一种纳米氧化铁修饰玻碳电极,并研究了镉离子在该修饰电极上的溶出伏安行为。结果表明,纳米氧化铁颗粒能有效促进镉离子的溶出伏安响应。在pH 6.0的磷酸缓冲溶液中,镉离子能有效吸附在纳米氧化铁表面并在-1.0 V时被还原。被还原的镉在正向扫描过程中可以重新氧化,并在-0.85 V处出现一明显的溶出伏安氧化峰。该峰电流随镉离子浓度的增大而增大,可用于对镉离子的检测。在最佳检测条件(pH 6.0,富集时间350 s,富集电位-1.0 V)下,镉离子的响应电流与其浓度在6.0×10-10～1.0×10-8mol/L以及1.0×10-8～1.0×10-5 mol/L范围内呈良好线性,检出限(S/N=3)为1.0×10-10 mol/L。干扰实验结果表明,一些常见的阳离子以及阴离子对镉离子的检测无明显干扰。将该方法用于实际样品的检测,回收率良好。     

α-Al2O3纳米粒子对Co-Ni合金异常共沉积电化学行为的影响

为了研究在电化学复合共沉积过程中,惰性纳米粒子和金属离子、电极表面的相互作用,以及由此产生的对合金电化学共沉积行为的影响.本文从两个吸附过程出发: 电解液中的金属离子和H＋在纳米粒子表面的吸附；纳米粒子迁移到阴极表面,在电极表面的吸附.采用Zeta电势和稳态极化以及电化学交流阻抗（EIS）研究了纳米Al2O3粒子和电解液中的金属离子，和电极表面的相互作用,进而分析了纳米粒子对Co2＋和Ni2＋还原沉积的影响规律.通过对阻抗数据的拟合,讨论了Al2O3纳米粒子对等效电路中各物理参数的影响.在H＋和不同金属离子在纳米粒子上发生竞争吸附的基础上,提出了纳米粒子和合金共沉积的可能反应历程.     

Colorimetric detection of Fe3+ ions using pyrophosphate functionalized gold nanoparticles

A sensitive, selective colorimetric Fe(3+) detection method has been developed by using pyrophosphate functionalized gold nanoparticles (P(2)O(7)(4-)-AuNPs). Gold nanoparticles were prepared by reducing HAuCl(4) with sodium borohydride, in the presence of Na(4)P(2)O(7). IR spectra suggested that pyrophosphates were capped on the surface of the gold nanoparticles. Aggregation of P(2)O(7)(4-)-AuNPs was induced immediately in the presence of Fe(3+) ions, yielding a color change from pink to violet. This Fe(3+)-induced aggregation of P(2)O(7)(4-)-AuNPs was monitored using first the naked eye and then UV-vis spectroscopy with a detection limit of 5.6 μM. The P(2)O(7)(4-)-AuNPs bound by Fe(3+) showed excellent selectivity compared to other metal ions (Ca(2+), Cd(2+), Co(2+), Fe(2+), Hg(2+), K(+), Mg(2+), Mn(2+), Na(+), Ni(2+), Pb(2+), and Zn(2+)). The best detection of Fe(3+) was achieved in a pH range from 3 to 9. In addition, the P(2)O(7)(4-)-AuNPs were also used to detect Fe(3+) in lake water samples, with low interference.     

Sm掺杂对Fe2O3催化剂NH3-SCR反应活性及抗水抗硫性作用

氮氧化物(NO_x)是主要的环境污染物之一,会造成酸雨、光化学烟雾和温室效应等环境问题.氨选择性催化还原(NH₃-SCR)技术是目前控制NO_x排放的最有效技术.其中,Fe₂O₃催化剂因其良好的抗硫性和低廉的成本而受到广泛关注,有望用作NO_x消除催化剂,但是它的低温还原性差、脱硝效率低等缺点限制了其应用.近期研究表明,钐掺杂金属氧化物可以调节其表面酸碱性及氧化还原性,可有效提高氧化物催化剂的脱硝效率和抗水抗硫性能.因此,将铁、钐二者优势结合,为合成一种低温、高效、环境友好型脱硝催化剂提供了可能.本文通过柠檬酸辅助的溶胶凝胶法合成了一系列钐均匀掺杂入Fe₂O₃纳米颗粒的复合氧化物脱硝催化剂,采用X射线光电子能谱(XPS),氢气程序升温还原(H2-TPR),氨气程序升温脱附(NH₃-TPD)以及原位漫反射红外光谱(in situ DRIFTS)等方法研究了钐的掺杂对铁基催化剂脱硝效率和抗水抗硫性的影响,旨在揭示催化剂表面物理化学性质和催化活性之间的关系.通过活性测试发现,钐的掺杂可以使Fe_0.94Sm_0.06O_x催化剂在175?325℃时实现>95%的脱硝效率和>93%的N2选择性.动力学测试研究表明,当基于催化剂的比表面积计算时,Fe_0.94Sm_0.06O_x催化剂的脱硝效率是纯Fe₂O₃催化剂的11倍;基于质量计算时则为37倍.此外,250℃时抗水抗硫测试结果显示,Fe_0.94Sm_0.06O_x催化剂可以在通入200×10^-6SO₂+5 vol%H₂O,空速为90000 h^-1时脱硝效率保持83%达168小时,并且在切断H₂O和SO₂后,该催化剂的脱硝效率可以很快完全恢复.XPS和H2-TPR结果表明,钐的掺杂使Fe_0.94Sm_0.06O_x催化剂的表面产生了大量的表面吸附氧,从而促进了NO的氧化以及快速NH₃-SCR反应的进行.NH₃-TPD与原位DRIFTS结果表明,钐的掺杂增强了催化剂的表面酸性,有效地提高了NH₃的吸附和活化能力,进而提高了催化剂的脱硝效率.另外,钐的掺杂还可以促使NH₄HSO₄在Fe_0.94Sm_0.06O_x催化剂表面较低温度下的分解,从而使催化剂具有很好的抗水抗硫性能.     

One-step synthesis of core(Cr)/shell(gamma-Fe(2)O(3)) nanoparticles

Core(Cr)/shell(gamma-Fe(2)O(3)) nanoparticles were synthesized by mixing Fe(CO)(5) and Cr(CO)(6) in the 9:1 ratio. These particles exhibit narrow size distribution with 13.5 nm as mean diameter and uniform spherical shape. The TEM image, which is in good agreement with the synchrotron powder XRD pattern, reveals the heterogeneous nature (core/shell structure). The analysis of the pattern reveals gamma-Fe(2)O(3) structure and a metal crystal structure. Mossbauer spectra, which support the superparamagnetic behavior determined by H-M measurement, do not show any traceable amount of Fe(0). This suggests that the metal component is Cr. EELS analysis and iron mapping suggest controlled stoichiometry and also confirm a core made of Cr and a shell made of gamma-Fe(2)O(3).     

钽掺杂六方相氧化钨对Sr2+的吸附:Zeta电位的测量及吸附机理的研究

研究Ta掺杂六方相氧化钨(hex-WO₃)材料在吸附Sr²⁺过程中其表面zeta电位的变化情况,并进一步探讨了吸附过程的热力学及吸附机理。结果表明：(1)在实验pH值范围内,Ta掺杂hex-WO₃悬浮液的zeta电位值随溶液中电解质的价态增大而增大;(2)且zeta电位随体系中离子强度的增加而增大;(3) Ta掺杂hex-WO₃对Sr²⁺的吸附容量随着温度降低而增大,随着离子强度的增加而减少;(4)吸附过程的吸附焓为－47 kJ·mol^-1,且Sr²⁺离子与材料表面之间主要为化学相互作用;(5) Ta掺杂hex-WO₃对Sr²⁺吸附过程主要为材料表面吸附及材料孔道内离子交换共同作用。     

Mechanism of pyridine-ligand exchanges at the different labile sites of 3d heterometallic and mixed valence mu3-oxo trinuclear clusters

The syntheses and single crystal X-ray structural analysis of five novel hetero- and homometallic mu 3-oxo trinuclear cluster with the formula [Fe (III) 2M (II)(mu 3-O)(mu-O 2CCH 3) 6(4-Rpy) 3]. x(4-Rpy). y(CH 3CN) where R = Ph for 1(Fe 2Mn), 2(Fe 2Fe), 3(Fe 2Co), 4(Fe 2Ni) and R = CF 3 for 5(Fe 2Co), are reported. The persistence of the structure for compounds 2- 5 in dichloromethane solution in the temperature range 190-320 K is demonstrated by (1)H and (19)F NMR spectroscopy. Even at the lowest temperature, the electron exchange in the homometallic mixed-valence compound 2(Fe 2Fe) is in the fast regime at the NMR time scale. Variable temperature and pressure NMR line broadening allowed quantifying the fast coordinated/free 4-Rpy exchanges at the two labile metal centers in these clusters: 2: Fe (III)( k (298)/10 (3) s (-1) = 16.6; Delta H (++) = 60.32 kJ mol (-1); Delta S (++) = + 34.8 J K (-1) mol (-1); Delta V (++) = + 12.5 cm (3) mol (-1)); 3: Fe (11.9; 58.92; +30.7; +10.6) and Co (2.8; 68.24; +49.8; +13.9); 4: Fe(12.2; 67.91; +61.0; -) and Ni (0.37; 78.62; +67.8; +12.3); 5: Fe (46; 58.21; +39.3; +14.2) and Co (4.7; 55.37; +11.2; +10.9). A limiting D mechanism is assigned to these exchange reactions. This assignment is based on a first-order rate law, the detection of intermediates, the positive and large entropies and volumes of activation. The order of reactivity k (Co) > k (Ni) is expected for a D mechanism at these metal centers: their low exchange rates are due to their strong binding with the 4-Rpy donor. Surrounded by oxygen donors the d (5) iron(III) usually reacts associatively; however, here due to low affinity of this ion for nitrogen the mechanism is D and the rate of exchange is very fast, even faster than on the divalent ions. There is no significant effect of the divalent ion in cluster 2, 3, and 5 on the exchange rates of 4-Phpy at the iron center, which seems to indicate that the specific electronic interactions between the three ions making the clusters do not influence the Fe (III)-N bond strength.     

Fast removal and recovery of Cr(VI) using surface-modified jacobsite (MnFe2O4) nanoparticles

In this work, the effectiveness of surface-modified jacobsite (MnFe2O4) nanoparticles was investigated for the removal and recovery of Cr(VI) from synthetic wastewater. Ten nanometer modified MnFe2O4 nanoparticles were produced to be a new adsorbent using a co-precipitation method followed by a surface redox reaction. The equilibrium time for Cr(VI) adsorption onto modified MnFe2O4 nanoparticles was as short as 5 min, and the adsorption data fit the Langmuir model well. The maximum uptake of 31.5 mg of Cr(VI)/g of modified MnFe2O4 was obtained at pH 2, which was comparable with other common adsorbents such as activated carbon and sawdust. The effects of ligands (EDTA, SO4(2-), NH4+) and ionic strength were studied in a pH range of 2-10. EDTA and SO4(2-) inhibited the adsorption of Cr(VI) over the entire pH range studied, whereas NH4+ enhanced the uptake of Cr(VI) at pH greater than 6.5. The mechanisms leading to Cr(VI) adsorption by modified MnFe2O4 nanoparticles were determined by X-ray diffraction and X-ray photoelectron spectroscopy to be a combination of electrostatic interaction and ion exchange. Regeneration studies indicated the potential reuse of the modified MnFe2O4 nanoparticles without sacrificing adsorption capacity and the possible recycling of Cr(VI) without changing the valence.     

"Nanocasting": using SBA-15 silicas as hard templates to obtain ultrasmall monodispersed gamma-Fe2O3 nanoparticles

This work describes the use of mesoporous SBA-15 silicas as hard templates for the size-controlled synthesis of oxide nanoparticles, with the pores acting as nanoscale reactors. This fundamental work is mainly aimed at understanding unresolved issues concerning the occurrence and size dependence of phase transitions in oxide nanocrystals. Aqueous solutions of Fe(NO3)3*9H2O are deposited inside the pores of SBA-15 silicas with mesopore diameters of 4.3, 6.6, and 9.5 nm. By calcination, the nitrate salt transforms into FeOx oxides. The XRD peaks of nanocrystals are broad and overlapping, resulting in ambiguities attributed to a given allotropic variety of Fe2O3 (alpha, epsilon, or gamma) or Fe3O4. The association of XRD, SAED, and Raman information is necessary to solve these ambiguities. The metastable gamma-Fe2O3 variety is selectively formed at low Fe/Si atomic ratio (ca. 0.20) and when a low calcination temperature is used (773 or 873 K followed by quenching to room temperature once the targeted temperature is reached). The small size dispersion of the patterned nanoparticles, suggested on a local scale by TEM, is confirmed statistically by magnetic measurements. The nanoparticles have a superparamagnetic behavior around room temperature. Their magnetic moments (from 220 to 370 mB), their sizes (from 4.0 to 4.8 nm), and their blocking temperatures (from 36 to 58 K) increase with the silica template mesopore diameter. Their magnetic properties are compared to those of standard gamma-Fe2O3 nanoparticles of similar size, obtained by coprecipitation in water and stabilized by a citrate coating.