Synthesis of Metastable Au-Fe Alloy Using Ordered Nanoporous Silica as a Hard Template

Nanoporous Au-Fe alloy was synthesized via a wet chemistry route using ordered nanoporous silica as a hard template. The nanoporous Au-Fe consisted of aligned arrays of nanopores that were uniform in composition and ordered in hexagonal lattice, whereas Au-Fe nanoparticles synthesized without templates exhibited broad dispersions in the chemical composition and/or particle size. Nanoporous Au-Fe has potential for applications as catalysts and/or adsorbents because of the large specific surface area of 81.2 m2·g−1 and high pore volume of 0.56 cm3·g−1.


Introduction
Alloy materials are widely used for day-to-day life.Copper-zinc alloys for corrosion-tolerant piping, gold-silver alloys for hardened jewelry wires, and titanium-aluminum alloys for biocompatible implants.Important infrastructures including bridges and railways are supported by hardened steels consisting of iron, chromium, and different metal additives.In the past few decades, alloy materials having nanometer pores and/or particle sizes, namely alloy nanomaterials, have been attracting attention because of their prominent performance as plasmonic sensors and catalysts for a broad range of energy/environmental applications [1].
Alloy nanomaterials are produced via either dry processes such as sputtering of alloy targets or wet chemistry routes including co-reduction of soluble metal precursors such as organometallic compounds and/or metal halides [2].The wet chemistry routes are superior to the dry processes in terms of the ability to precisely control the pore-size-and/or particle-size distribution [3].Monodispersed Au-Ag nanoparticles were prepared by co-reduction of Gold [1] chloride trihydrate and silver acetate in aqueous solution containing oleylamine as a surfactant, of which particle-size distribution was within 6 nm to 13 nm [4].Nanoporous Pd-Pt alloy materials were synthesized in aqueous solution by soft-template methods, where self-assembled micelles served as a soft template to realize a monodispersed pore diameter [5].
However, current wet chemistry routes face a challenge of low chemical homogeneity of the targeted alloy nanomaterials.The wet chemistry synthesized alloy nanomaterials are prone to phase segregation especially when the constituent metals are much different in reduction potentials: one of the metal precursors with a low reduction potential readily forms a core, on which the other metal precursor with a high reduction potential precipitates to develop core-shell structures [6].Post-annealing of such as-prepared alloy nanomaterials can improve the compositional uniformity because of promoted atomic interdiffusion but often lead to uncontrollable broadening in the particle/pore-size distribution due to particle agglomeration [7,8].Moreover, nanomaterials of thermodynamically unstable (metastable) alloys are hardly materialized by post-annealing because phase separations can be promoted more than atomic interdiffusion at elevated temperatures.
Herein, we report that ordered nanoporous silica materials (SBA15) can serve as an efficient hard template for the synthesis of metastable alloy nanomaterials consisting of metal elements with widely different reduction potentials (Scheme 1) [9].The nanoporous template absorbs and disperses different metal precursors on the pore surface.The dispersed metal precursors are then reduced with chemical reagents to precipitate uniform alloy layers over the template.The nanoporous template is finally dissolved in solution to yield the desired alloy with a skeletal porous structure replicated from the template.
annealing of such as-prepared alloy nanomaterials can improve the compositional uniformity because of promoted atomic interdiffusion but often lead to uncontrollable broadening in the particle/pore-size distribution due to particle agglomeration [7,8].Moreover, nanomaterials of thermodynamically unstable (metastable) alloys are hardly materialized by post-annealing because phase separations can be promoted more than atomic interdiffusion at elevated temperatures.
Herein, we report that ordered nanoporous silica materials (SBA15) can serve as an efficient hard template for the synthesis of metastable alloy nanomaterials consisting of metal elements with widely different reduction potentials (Scheme 1) [9].The nanoporous template absorbs and disperses different metal precursors on the pore surface.The dispersed metal precursors are then reduced with chemical reagents to precipitate uniform alloy layers over the template.The nanoporous template is finally dissolved in solution to yield the desired alloy with a skeletal porous structure replicated from the template.We demonstrate that this hard-template synthesis is successfully applicable to the gold (Au)iron [10] alloy system although it is acknowledged that Au1−xFex (0.5 < x < 0.9) is a thermodynamically metastable alloy that exhibits a phase separation at 750 °C or higher temperatures in ambient condition [11,12].Gold chloride (AuCl3, Sigma-Aldrich Japan, Tokyo, Japan) and iron acetylacetonate (Fe(II)acac) (Sigma-Aldrich Japan, Tokyo, Japan) were first dissolved in aprotic solvent containing SBA15 powder.Superhydride (Lithium Triethylborohydride in tetrahydrofuran) (Sigma-Aldrich Japan, Tokyo, Japan) was then added to the solution to reduce the Au(III)-and Fe(II) precursors and precipitate nanometer-thick Au-Fe alloy layers over the pore surface of the template.The template was finally dissolved in hydrofluoric acid solution (Sigma-Aldrich Japan, Tokyo, Japan) to obtain the targeted Au-Fe nanomaterials having a nanoporous structure and large specific surface area.Powder X-ray diffractometry (pXRD) (Malvern PANalytical, Tokyo, Japan), hard X-ray photoemission spectroscopy (HAXPES) (Scienta Omicron, Tokyo, Japan), Brunauer-Emmett-Teller (BET) (Micromeritics Kashiwa, Japan) physisorption and scanning transmission electron microscopy (STEM) (JEOL, Tokyo, Japan) demonstrated that the obtained Au-Fe alloy nanomaterials had a uniform composition and an ordered nanoporous structure, which was inherited from the SBA15 hard template.

Sample Preparation
An aliquot of 200 mg of SBA-15 was added to 35 mL of diethyleneglycol dimethyl (Sigma-Aldrich Japan, Tokyo, Japan) ether (diglyme) (Sigma-Aldrich Japan, Tokyo, Japan) and continuously stirred for 6 h, then 50 mg of AuCl3 and 106 mg of Fe(II)acac were added to the solution and stirred for 1 h more.An aliquot of 2.2 mL of super hydride (1 M in tetrahydrofuran) (Sigma-Aldrich Japan, Tokyo, Japan) was finally added to the solution.The color of the solution turned from dark brown to turbid black upon addition of super hydride.The solution was allowed for stirring overnight and centrifuged to collect black powder as the final product.All of these procedures were carried out in dry Ar atmosphere in a glove box (mBraun, Garching, Germany).The collected black powder was We demonstrate that this hard-template synthesis is successfully applicable to the gold (Au)-iron [10] alloy system although it is acknowledged that Au 1−x Fe x (0.5 < x < 0.9) is a thermodynamically metastable alloy that exhibits a phase separation at 750 • C or higher temperatures in ambient condition [11,12].Gold chloride (AuCl 3 , Sigma-Aldrich Japan, Tokyo, Japan) and iron acetylacetonate (Fe(II)acac) (Sigma-Aldrich Japan, Tokyo, Japan) were first dissolved in aprotic solvent containing SBA15 powder.Superhydride (Lithium Triethylborohydride in tetrahydrofuran) (Sigma-Aldrich Japan, Tokyo, Japan) was then added to the solution to reduce the Au(III)-and Fe(II) precursors and precipitate nanometer-thick Au-Fe alloy layers over the pore surface of the template.The template was finally dissolved in hydrofluoric acid solution (Sigma-Aldrich Japan, Tokyo, Japan) to obtain the targeted Au-Fe nanomaterials having a nanoporous structure and large specific surface area.Powder X-ray diffractometry (pXRD) (Malvern PANalytical, Tokyo, Japan), hard X-ray photoemission spectroscopy (HAXPES) (Scienta Omicron, Tokyo, Japan), Brunauer-Emmett-Teller (BET) (Micromeritics Kashiwa, Japan) physisorption and scanning transmission electron microscopy (STEM) (JEOL, Tokyo, Japan) demonstrated that the obtained Au-Fe alloy nanomaterials had a uniform composition and an ordered nanoporous structure, which was inherited from the SBA15 hard template.

Sample Preparation
An aliquot of 200 mg of SBA-15 was added to 35 mL of diethyleneglycol dimethyl (Sigma-Aldrich Japan, Tokyo, Japan) ether (diglyme) (Sigma-Aldrich Japan, Tokyo, Japan) and continuously stirred for 6 h, then 50 mg of AuCl 3 and 106 mg of Fe(II)acac were added to the solution and stirred for 1 h more.An aliquot of 2.2 mL of super hydride (1 M in tetrahydrofuran) (Sigma-Aldrich Japan, Tokyo, Japan) was finally added to the solution.The color of the solution turned from dark brown to turbid black upon addition of super hydride.The solution was allowed for stirring overnight and centrifuged to collect black powder as the final product.All of these procedures were carried out in dry Ar atmosphere in a glove box (mBraun, Garching, Germany).The collected black powder was exposed to air, washed with diglyme (Sigma-Aldrich Japan, Tokyo, Japan) and hexane (Sigma-Aldrich Japan, Tokyo, Japan), and finally heated at 700 • C in a vacuum-sealed quartz tube for 6.5 h.The sample was immersed in 5% aqueous HF solution (Sigma-Aldrich Japan, Tokyo, Japan) for 20 min to dissolve SBA15, washed with ethanol (Sigma-Aldrich Japan, Tokyo, Japan), and dried in air at 60 • C for 12 h.

Characterization
The prepared sample was examined using a powder X-ray diffractometer (X'Pert-PRO, Malvern PANalytical, Tokyo, Japan) with Cu Kα radiation (λ = 0.154060 nm).A non-reflection Si plate was used as a sample holder to reduce the background.Hitachi SU8200 scanning electron microscope equipped with an X-ray energy-dispersive spectrometer (Hitachi, Tokyo, Japan) was used for microscopic characterization and elemental compositions at an accelerating voltage of 5 kV and 15 kV, respectively.High-resolution transmission electron microscopic (HRTEM) images were obtained with a JEOL 2100-F2 microscope (JEOL, Tokyo, Japan).

Results and Discussions
Figure 1a shows the pXRD profiles of the prepared nanoporous Au-Fe alloy (20 < 2θ < 90 degrees).The major reflection peaks were positioned at 2θ = 38.80,45.10, 65.60, 78.90, and 83.06 degrees, which corresponded to the (111)-, (200)-, (220)-, (311)-, and (222) of the face-centered cubic structure (a = 0.3980 ± 0.0002 nm), respectively.The chemical composition of the Au-Fe alloy was estimated as Au 0.75 Fe 0.25 because the calculated lattice parameter was consistent with the reported value of an atomically ordered Au 3 Fe alloy [10].The peak positions were shifted toward the higher reflection angles with respect to the corresponding peaks from pure Au at 38.10, 44.30, 64.50, 77.50, and 81.60 degrees (a = 0.4070 ± 0.0002 nm), showing that the crystal lattice of the nanoporous Au-Fe was 2.2% smaller than that of pure Au due to the incorporation of Fe atoms in the Au matrix [10].The low-angle pXRD profile of the nanoporous Au-Fe (0.70 < 2θ < 2.0 degrees) is presented in Figure 1b.There are two peaks observed at 2θ = 0.80 and 1.30 degrees, corresponding to the (100)-and (110) reflections of a hexagonally ordered nanopores replicated from SBA15, of which lattice parameter along the hexagonal edge was calculated as 13.20 ± 0.02 nm [13,14].
exposed to air, washed with diglyme (Sigma-Aldrich Japan, Tokyo, Japan) and hexane (Sigma-Aldrich Japan, Tokyo, Japan), and finally heated at 700 °C in a vacuum-sealed quartz tube for 6.5 h.The sample was immersed in 5% aqueous HF solution (Sigma-Aldrich Japan, Tokyo, Japan) for 20 min to dissolve SBA15, washed with ethanol (Sigma-Aldrich Japan, Tokyo, Japan), and dried in air at 60 °C for 12 h.

Characterization
The prepared sample was examined using a powder X-ray diffractometer (X'Pert-PRO, Malvern PANalytical, Tokyo, Japan) with Cu Kα radiation (λ = 0.154060 nm).A non-reflection Si plate was used as a sample holder to reduce the background.Hitachi SU8200 scanning electron microscope equipped with an X-ray energy-dispersive spectrometer (Hitachi, Tokyo, Japan) was used for microscopic characterization and elemental compositions at an accelerating voltage of 5 kV and 15 kV, respectively.High-resolution transmission electron microscopic (HRTEM) images were obtained with a JEOL 2100-F2 microscope (JEOL, Tokyo, Japan).

Results and Discussions
Figure 1a shows the pXRD profiles of the prepared nanoporous Au-Fe alloy (20 < 2θ < 90 degrees).The major reflection peaks were positioned at 2θ = 38.80,45.10, 65.60, 78.90, and 83.06 degrees, which corresponded to the (111)-, (200)-, (220)-, (311)-, and (222) of the face-centered cubic structure (a = 0.3980 ± 0.0002 nm), respectively.The chemical composition of the Au-Fe alloy was estimated as Au0.75Fe0.25 because the calculated lattice parameter was consistent with the reported value of an atomically ordered Au3Fe alloy [10].The peak positions were shifted toward the higher reflection angles with respect to the corresponding peaks from pure Au at 38.10, 44.30, 64.50, 77.50, and 81.60 degrees (a = 0.4070 ± 0.0002 nm), showing that the crystal lattice of the nanoporous Au-Fe was 2.2% smaller than that of pure Au due to the incorporation of Fe atoms in the Au matrix [10].The lowangle pXRD profile of the nanoporous Au-Fe (0.70 < 2θ < 2.0 degrees) is presented in Figure 1b.There are two peaks observed at 2θ = 0.80 and 1.30 degrees, corresponding to the (100)-and (110) reflections of a hexagonally ordered nanopores replicated from SBA15, of which lattice parameter along the hexagonal edge was calculated as 13.20 ± 0.02 nm [13,14].Figure 2 presents the nitrogen (N2) adsorption-desorption isotherms of the nanoporous Au-Fe.An aliquot of 100 mg of sample was degassed at 150 °C for 3 h, then the N2-adsorption/desorption isotherm was carried out using Micrometrics ASAP 2020 (National Institute for Materials Science (NIMS), Tsukuba, Japan).The specific surface area of the nanoporous Au-Fe was calculated as 81.2 m 2 •g −1 from the N2 adsorption-desorption isotherms.The pore-size distribution curve (inset in Figure 2) had a maximum at 18 nm, which was consistent with the lattice constant (13.2 nm) of the ordered nanopores that was determined by low-angle pXRD (Figure 1b).The mean value of the pore diameter was calculated from the N2 adsorption data as 24 ± 2 nm. Figure 2 presents the nitrogen (N 2 ) adsorption-desorption isotherms of the nanoporous Au-Fe.An aliquot of 100 mg of sample was degassed at 150 • C for 3 h, then the N 2 -adsorption/desorption isotherm was carried out using Micrometrics ASAP 2020 (National Institute for Materials Science (NIMS), Tsukuba, Japan).The specific surface area of the nanoporous Au-Fe was calculated as 81.2 m 2 •g −1 from the N 2 adsorption-desorption isotherms.The pore-size distribution curve (inset in Figure 2) had a maximum at 18 nm, which was consistent with the lattice constant (13.2 nm) of the ordered nanopores that was determined by low-angle pXRD (Figure 1b).The mean value of the pore diameter was calculated from the N 2 adsorption data as 24 ± 2 nm.The successful replication of the nanoporous structure of the SBA15 template to the nanoporous Au-Fe was confirmed by microscopic characterizations including SEM and TEM (Figure S1). Figure 2a shows that the prepared nanoporous Au-Fe had a rod-like shape with an average thickness of 1 ± 0.1 μm, having a number of pores over the surface (Figure 2b).High-resolution TEM observation demonstrated that the porous structure was developed not only over the surface but also in the bulk, of which pore diameter was 10-20 nm (Figure 2c).Compositional mapping showed that Au and Fe were uniformly distributed in the nanoporous Au-Fe (the sample was finely pulverized by sonication in ethanol to improve electron-beam transmission for quantitative analysis).Based on the results of pXRD, SEM (JEOL, Tokyo, Japan) and TEM (JEOL, Tokyo, Japan) characterizations, we conclude that the nanoporous Au-Fe comprised hexagonally ordered nanopore arrays of Au0.75Fe0.25 alloy (see Figures S2-S5, Table 1 for HAXPES and EDS data).We further estimated the composition of the Au-Fe nanomaterials from the HAXPES peak intensities of metallic Au 4f-and Fe 2p emissions (Figures S2-S4).The calculated Au/Fe ratio was 2.42 ± 0.24 (see supporting information), which is fairly consistent with the value estimated from pXRD: Au/Fe = 0.75/0.25 = 3.The successful replication of the nanoporous structure of the SBA15 template to the nanoporous Au-Fe was confirmed by microscopic characterizations including SEM and TEM (Figure S1). Figure 3a shows that the prepared nanoporous Au-Fe had a rod-like shape with an average thickness of 1 ± 0.1 µm, having a number of pores over the surface (Figure 3b).High-resolution TEM observation demonstrated that the porous structure was developed not only over the surface but also in the bulk, of which pore diameter was 10-20 nm (Figure 3c).Compositional mapping showed that Au and Fe were uniformly distributed in the nanoporous Au-Fe (the sample was finely pulverized by sonication in ethanol to improve electron-beam transmission for quantitative analysis).Based on the results of pXRD, SEM (JEOL, Tokyo, Japan) and TEM (JEOL, Tokyo, Japan) characterizations, we conclude that the nanoporous Au-Fe comprised hexagonally ordered nanopore arrays of Au 0.75 Fe 0.25 alloy (see Figures S2-S5, Table 1 for HAXPES and EDS data).We further estimated the composition of the Au-Fe nanomaterials from the HAXPES peak intensities of metallic Au 4f -and Fe 2p emissions (Figures S2-S4).The calculated Au/Fe ratio was 2.42 ± 0.24 (see supporting information), which is fairly consistent with the value estimated from pXRD: Au/Fe = 0.75/0.25 = 3.To verify how the hard template contributed to the homogeneity and porous nature of the nanoporous Au-Fe, free-standing Au-Fe nanoparticles were synthesized without SBA15 by coreduction of the same precursors in the same condition as for the nanoporous Au-Fe.AuCl3 and Fe(II)acac were dissolved in diglyme containing no SBA15 and reduced with superhydride to yield black, fine powder of Au-Fe nanoparticles.The Au-Fe nanoparticles were post-annealed in vacuum at 700 °C as was the nanoporous Au-Fe.The obtained nanoparticles were widely distributed in size from 20 nm to 100 nm (Figure 4a).Moreover, the chemical composition of the prepared nanoparticles was not uniform: each of the individual alloy nanoparticles had different Au and Fe contents (Figure 4b).To verify how the hard template contributed to the homogeneity and porous nature of the nanoporous Au-Fe, free-standing Au-Fe nanoparticles were synthesized without SBA15 by co-reduction of the same precursors in the same condition as for the nanoporous Au-Fe.AuCl 3 and Fe(II)acac were dissolved in diglyme containing no SBA15 and reduced with superhydride to yield black, fine powder of Au-Fe nanoparticles.The Au-Fe nanoparticles were post-annealed in vacuum at 700 • C as was the nanoporous Au-Fe.The obtained nanoparticles were widely distributed in size from 20 nm to 100 nm (Figure 4a).Moreover, the chemical composition of the prepared nanoparticles was not uniform: each of the individual alloy nanoparticles had different Au and Fe contents (Figure 4b).To verify how the hard template contributed to the homogeneity and porous nature of the nanoporous Au-Fe, free-standing Au-Fe nanoparticles were synthesized without SBA15 by coreduction of the same precursors in the same condition as for the nanoporous Au-Fe.AuCl3 and Fe(II)acac were dissolved in diglyme containing no SBA15 and reduced with superhydride to yield black, fine powder of Au-Fe nanoparticles.The Au-Fe nanoparticles were post-annealed in vacuum at 700 °C as was the nanoporous Au-Fe.The obtained nanoparticles were widely distributed in size from 20 nm to 100 nm (Figure 4a).Moreover, the chemical composition of the prepared nanoparticles was not uniform: each of the individual alloy nanoparticles had different Au and Fe contents (Figure 4b).

Scheme 1 .
Scheme 1. Hard-template synthesis of skeletal nanoporous alloys via wet chemistry routes.

Scheme 1 .
Scheme 1. Hard-template synthesis of skeletal nanoporous alloys via wet chemistry routes.

Figure 1 .
Figure 1.pXRDprofiles for Au nanoparticles and the nanoporous Au-Fe alloy in high reflection (a) and low reflection angles (b).

Figure 1 .
Figure 1.pXRD profiles for Au nanoparticles and the nanoporous Au-Fe alloy in high reflection (a) and low reflection angles (b).

Figure 3 .
Figure 3. (a) Scanning electron microscope (SEM); and (b) transmission electron microscope (TEM) images of the nanoporous Au-Fe (c).An annular-dark field (HAADF) image and compositional mapping images of the nanoporous Au-Fe corresponding to Au, Fe, and a merged image (d).The scale bar in (d) corresponds to 10 nm.

Figure 4 .
Figure 4. (a) A transmission electron microscope (STEM) image annular-dark field (HAADF) image and (b) compositional mapping images of the Au-Fe nanoparticles synthesized without templates.The red scale bar in (b) corresponds to 50 nm.

Figure 3 .
Figure 3. (a) Scanning electron microscope (SEM); and (b) transmission electron microscope (TEM) images of the nanoporous Au-Fe (c).An annular-dark field (HAADF) image and compositional mapping images of the nanoporous Au-Fe corresponding to Au, Fe, and a merged image (d).The scale bar in (d) corresponds to 10 nm.

Figure 3 .
Figure 3. (a) Scanning electron microscope (SEM); and (b) transmission electron microscope (TEM) images of the nanoporous Au-Fe (c).An annular-dark field (HAADF) image and compositional mapping images of the nanoporous Au-Fe corresponding to Au, Fe, and a merged image (d).The scale bar in (d) corresponds to 10 nm.

Figure 4 .
Figure 4. (a) A transmission electron microscope (STEM) image annular-dark field (HAADF) image and (b) compositional mapping images of the Au-Fe nanoparticles synthesized without templates.The red scale bar in (b) corresponds to 50 nm.

Figure 4 .
Figure 4. (a) A transmission electron microscope (STEM) image annular-dark field (HAADF) image and (b) compositional mapping images of the Au-Fe nanoparticles synthesized without templates.The red scale bar in (b) corresponds to 50 nm.