Intra-wire alloy homogeneity and the origin of the enhanced magnetic susceptibility of necklace-shaped bimetallic nanowires templated by a hybrid organic–inorganic mesoporous material (HMM-1)

Necklace-shaped Pt, Rh, Pt–Rh, and Pt–Pd nanowires of about 3 nm diameter can be grown by ultraviolet photoreduction of impregnated chlorometal precursors inside the one-dimensional channels of HMM-1, a hybrid organic–inorganic mesoporous material with ethylene-bridged framework. The originating 2001 report raised two characterization questions that no published study has since resolved: (1) whether the bimetallic (Pt–Rh, Pt–Pd) necklace nanowires are compositionally homogeneous random alloys or show segregation within individual wires — the alloy claim rests on lattice-fringe spacings and on energy-dispersive X-ray analysis with a probe larger than the wire, and only preliminary extended X-ray absorption fine structure (EXAFS) evidence for Pt–Rh bonding was reported; and (2) what quantitatively causes the strongly enhanced, low-temperature magnetic susceptibility of the Pt–Pd wires — two to three times the sum of bulk Pt and Pd susceptibilities, rising below about 90 K with a deviation from Curie's law near 60 K — tentatively attributed to the low-dimensional necklace topology but never modeled, replicated, or checked against host diamagnetic contributions. Later work by the same group settled the wire-growth mechanism, extended the necklace morphology to other metals and to much longer wires, and applied such wires in catalysis, but did not resolve either question. The problem is to determine the intra-wire structure (alloy homogeneity) and the quantitative physical origin of the magnetic enhancement of these specific mesopore-confined bimetallic nanowires.

Background

Ordered mesoporous silicas such as MCM-41, SBA-15, and FSM-16 are templated inorganic frameworks permeated by parallel one-dimensional channels of roughly 2–10 nm diameter, and since the 1990s they have been used as nanoscale molds for metals: a volatile or solution-borne metal precursor is impregnated into the channels and reduced in place, so that metal grows confined to the channel geometry (often called a ship-in-bottle or templating synthesis). HMM-1 is a hybrid organic–inorganic analogue, a periodic mesoporous organosilica in which every framework bridge between silicon centers is an ethylene group ($-\mathrm{CH_2CH_2}-$) instead of oxygen, giving a 2D hexagonal array of 3.1 nm channels with hydrophobic organic walls. Fukuoka, Sakamoto, Guan, Inagaki, Sugimoto, Fukushima, Hirahara, Iijima, and Ichikawa (J. Am. Chem. Soc. 123, 3373–3374, 2001) showed that impregnating HMM-1 with aqueous $\mathrm{H_2PtCl_6}$, $\mathrm{RhCl_3}$, or $\mathrm{H_2PdCl_4}$ (or their mixtures), adsorbing water and methanol vapor, and irradiating with a mercury lamp for 24 h yields metallic nanowires confined to the channels: Pt, Rh, Pt–Rh (charged atomic ratio $\mathrm{Pt/Rh}=1.0$), and Pt–Pd (charged $\mathrm{Pt/Pd}=6.1$). Unlike the smooth, rodlike wires obtained in purely siliceous hosts such as FSM-16, the HMM-1 wires have a necklace (beaded) morphology — connected particles of 3–5 nm length with 3.1 nm maximum and 1.8–2.0 nm minimum diameter — which the authors tentatively attributed to the weak metal interaction with the hydrophobic organic walls; the morphology survives heating to 473 K under vacuum. Two characterizations in that paper were explicitly incomplete. First, energy-dispersive X-ray (EDX) analysis, whose ~5 nm spot averages over the 3 nm wire plus surrounding host, gave Pt/Rh ratios of 1.2–1.3 and Pt/Pd ratios of 6.4–7.3, close to the feed ratios, and high-resolution transmission electron microscopy (HRTEM) lattice fringes ($d \approx 0.23$ nm) fell between the face-centered-cubic (fcc) Pt(111) and Rh(111) spacings; a footnote acknowledged that EXAFS (extended X-ray absorption fine structure — the element-specific oscillations beyond an X-ray absorption edge, which give coordination numbers and neighbor distances for the absorbing atom and can therefore distinguish heterometal bonding from phase segregation) gave only preliminary evidence of Pt–Rh bonding, with a detailed study in progress. Whether an individual bimetallic wire is a homogeneous random alloy or slightly segregated was thus left open, and it matters because alloy homogeneity controls the electronic and catalytic properties of bimetallic nanowires. Second, magnetic susceptibility (magnetization per unit applied field) measured from ~5 K to 300 K showed that Pt–Pd/HMM-1 increases strongly below ~90 K, reaching two to three times the value expected from the sum of bulk Pt and bulk Pd susceptibilities, with a noted deviation from Curie's law ($\chi \propto 1/T$, the expectation for independent localized moments) near 60 K; the authors tentatively attributed the enhancement to the low-dimensional necklace topology without a quantitative model. The physical background makes a confinement or size effect plausible but not automatic: bulk Pd is a nearly ferromagnetic paramagnet (its spin susceptibility is strongly amplified by the exchange-driven Stoner enhancement, which keeps the metal just short of developing a spontaneous ferromagnetic moment) whose susceptibility is known to change strongly with particle size in dispersed small Pd particles (Kubicka, 1966; Ladas, 1978), and first-principles work later showed that atomically thin Pd nanowires and contacts do develop spontaneous magnetization (Delin, Tosatti, and Weht, 2004; Stewart, 2007) — but those results concern atomic-scale or unsupported systems, not ~3 nm Pt-rich wires embedded in a diamagnetic organosilica host whose contribution, if not subtracted (a standard diamagnetic correction), could distort the measured signal of a 5 wt% metal loading. No subsequent measurement or theory has addressed the Pt–Pd/HMM-1 data itself. The same group's subsequent papers resolved related issues around this system: the photoreduction growth mechanism (time-resolved TEM/XRD/EXAFS showing Pt-ion migration and reduction kinetics), the extraction and length scaling of the wires (up to 25 μm, aspect ratio ~8300), extension of the necklace morphology to gold, and catalytic application of the wires — which is why those parts of the original open-issue list are not part of the surviving problem.

Problem Statement

For the necklace-shaped bimetallic nanowires (~3 nm diameter, connected 3–5 nm particles) grown by photoreduction inside the 3.1 nm channels of the ethylene-bridged hybrid mesoporous material HMM-1 — the two unresolved characterization questions raised by the originating 2001 report of Fukuoka, Sakamoto, Guan, Inagaki, Sugimoto, Fukushima, Hirahara, Iijima, and Ichikawa that remain open: (1) Determine the compositional structure of individual Pt–Rh/HMM-1 and Pt–Pd/HMM-1 wires: are they homogeneous random alloys at the single-wire level, or is there intrawire segregation (core–shell or domain structure)? The reported evidence does not resolve this: the ~5 nm-spot EDX averages over the ~3 nm wire and the surrounding host, and a single HRTEM d-spacing cannot separate a homogeneous random alloy from partial segregation; the degree of alloying (heterometal coordination numbers relative to a random alloy of the same composition) therefore remains to be determined quantitatively. (2) Establish quantitatively the origin of the enhanced magnetic susceptibility of Pt–Pd/HMM-1: after applying a proper diamagnetic correction for the HMM-1 host, determine whether the reported two- to three-fold enhancement over the sum of bulk Pt and Pd susceptibilities, its rise below ~90 K, and the deviation from Curie's law near 60 K are intrinsic to the confined, low-dimensional, Pd-containing necklace wires or arise from host, impurity, or measurement effects, and provide a model that reproduces the measured temperature dependence between ~5 and 300 K. The further directions the source paper raised — extension of the necklace morphology to other metals, wire-length scalability, and catalytic application — were subsequently addressed in follow-up work and are not part of this problem.

References

1. A. Fukuoka, Y. Sakamoto, S. Guan, S. Inagaki, N. Sugimoto, Y. Fukushima, K. Hirahara, S. Iijima, M. Ichikawa, "Novel Templating Synthesis of Necklace-Shaped Mono- and Bimetallic Nanowires in Hybrid Organic−Inorganic Mesoporous Material", Journal of the American Chemical Society 123, 3373–3374 (2001), DOI 10.1021/ja004067y, https://doi.org/10.1021/ja004067y.
2. Y. Sakamoto, A. Fukuoka, T. Higuchi, N. Shimomura, S. Inagaki, M. Ichikawa, "Synthesis of Platinum Nanowires in Organic−Inorganic Mesoporous Silica Templates by Photoreduction: Formation Mechanism and Isolation", The Journal of Physical Chemistry B 108, 853–858 (2004), DOI 10.1021/jp036617v, https://doi.org/10.1021/jp036617v.
3. A. Fukuoka, N. Higashimoto, Y. Sakamoto, S. Inagaki, Y. Fukushima, M. Ichikawa, "Preparation, XAFS Characterization, and Catalysis of Platinum Nanowires and Nanoparticles in Mesoporous Silica FSM-16", Topics in Catalysis 18, 73–78 (2002), DOI 10.1023/A:1013886301858, https://doi.org/10.1023/A:1013886301858.
4. A. Fukuoka, M. Ichikawa, "Nanoscale fabrication of metal particles and wires using mesoporous silica templates: Electronic properties and catalytic performances in PROX reaction", Topics in Catalysis 40, 103–109 (2006), DOI 10.1007/s11244-006-0112-8, https://doi.org/10.1007/s11244-006-0112-8.
5. A. Fukuoka, Y. Sakamoto, T. Higuchi, N. Shimomura, M. Ichikawa, "Synthesis and electronic property of platinum nanowire and nanoparticle in mesoporous silica template", Journal of Porous Materials 13, 231–235 (2006), DOI 10.1007/S10934-006-8009-2, https://doi.org/10.1007/S10934-006-8009-2.
6. A. Fukuoka, T. Higuchi, T. Ohtake, T. Oshio, J. Kimura, Y. Sakamoto, N. Shimomura, S. Inagaki, M. Ichikawa, "Nanonecklaces of Platinum and Gold with High Aspect Ratios Synthesized in Mesoporous Organosilica Templates by Wet Hydrogen Reduction", Chemistry of Materials 18, 337–343 (2006), DOI 10.1021/cm0516964, https://doi.org/10.1021/cm0516964.
7. P. L. Dhepe, A. Fukuoka, M. Ichikawa, "Novel fabrication and catalysis of nano-structured Rh and RhPt alloy particles occluded in ordered mesoporous silica templates using supercritical carbon dioxide", Physical Chemistry Chemical Physics 5, 5565 (2003), DOI 10.1039/B310261K, https://doi.org/10.1039/B310261K.
8. P. L. Dhepe, A. Fukuoka, M. Ichikawa, "Preparation of highly dispersed RhPt alloy catalysts in mesoporous silica using supercritical carbon dioxide and selective synthesis of ethane in butane hydrogenolysis", Chemical Communications, 590–591 (2003), DOI 10.1039/B211680D, https://doi.org/10.1039/B211680D.
9. A. Fukuoka, N. Higashimoto, Y. Sakamoto, S. Inagaki, Y. Fukushima, M. Ichikawa, "Preparation and catalysis of Pt and Rh nanowires and particles in FSM-16", Microporous and Mesoporous Materials 48, 171–179 (2001), DOI 10.1016/S1387-1811(01)00341-9, https://doi.org/10.1016/S1387-1811(01)00341-9.
10. "22-O-05 Template synthesis and catalysis of bimetallic platinum-rhodium and -palladium nanowires in mesoporous materials", Studies in Surface Science and Catalysis (13th International Zeolite Conference proceedings) (2001), DOI 10.1016/S0167-2991(01)81303-3, https://doi.org/10.1016/S0167-2991(01)81303-3.
11. A. Delin, E. Tosatti, R. Weht, "Magnetism in Atomic-Size Palladium Contacts and Nanowires", Physical Review Letters 92, 057201 (2004), DOI 10.1103/PhysRevLett.92.057201, https://doi.org/10.1103/PhysRevLett.92.057201.
12. D. A. Stewart, "Magnetism in coaxial palladium nanowires", Journal of Applied Physics 101, 09D503 (2007), DOI 10.1063/1.2672303, https://doi.org/10.1063/1.2672303.
13. H. Kubicka, "The magnetic susceptibility of dispersed palladium", Journal of Catalysis 5, 39–43 (1966), DOI 10.1016/S0021-9517(66)80123-9, https://doi.org/10.1016/S0021-9517(66)80123-9.
14. S. Ladas, "The magnetic susceptibility of small palladium particles", Journal of Catalysis 53, 356–365 (1978), DOI 10.1016/0021-9517(78)90107-0, https://doi.org/10.1016/0021-9517(78)90107-0.