Communication: Chemisorption of muonium on gold nanoparticles: A sensitive new probe of surface magnetism and reactivity

Abstract

Chemisorption of muonium onto the surface of gold nanoparticles has been observed. Muonium (μ+e−), a light hydrogen-like atom, reacts chemically with uncapped 7 nm gold nanoparticles embedded in mesoporous silica (SBA-15) with a strong temperature-dependent rate. The addition rate is fast enough to allow coherent spin transfer into a diamagnetic muon state on the nanoparticle surface. The muon is well established as a sensitive probe of static or slowly fluctuating magnetic fields in bulk matter. These results represent the first muon spin rotation signal on a nanoparticle surface or any metallic surface. Only weak magnetic effects are seen on the surface of these Au nanoparticles consistent with Pauli paramagnetism.

Dynamics of Anisotropic Muonium on Silica Surfaces Explained by Monte Carlo Simulation of the Muon Depolarization

“…Below 25 K, λ TF increases sharply, before it plateaus below 7 K. The small peak around 40 K is attributed to trapping on the silica surface, as reported earlier in non-porous silica powders [5]. Note that this peak was not observed in our previous study [2], performed on the same compound, but a different batch. This suggests that small variations in the preparation or bake-out procedure may introduce more trapping sites.…”

“…In this contribution we report on muon spin rotation (µSR) data taken in mesoporous silica (SBA-15) at low temperatures. There has been renewed interest in µSR studies in silica (SiO 2 ) caused by (1) efforts to create a high-intensity source of muonium (Mu) in vacuum that may be ionized to create an intense beam of ultra slow muons for particle physics and material science applications [1], and (2) the investigation of magnetic properties of metal nanoparticles embedded in silica [2]. Previous studies have shown that a large fraction of muonium, the hydrogen-like bound state of a positive muon and an electron, is formed and efficiently escapes the silica grains into the mesopores, where it thermalizes [3,4].…”

Observation of a Charge-Neutral Muon-Polaron Complex in Antiferromagnetic Cr2O3

“…Under the assumption that B ext does not induce changes of B int , the resulting frequency multiplets can Here we describe a model for the E2→E1 transition assuming a thermally activated, exponential rate of the form Λ(T ) = ν 0 exp(−E a /k B T ), where E a and ν 0 are activation energy and attempt frequency. The following expression describes the observable signal precessing at f E1 (compare [65,66]):…”

Level Crossing Resonance Studies of the Muoniated Cyclohexadienyl Radical (MuĊ6H6) Interacting with Uncapped Gold Nanoparticles in Porous Silica Hosts

“…For reference, the highest loading in the present study (Table 1) is comparable. Note the loss of the Δ1 resonance in the upper scan for the bare silica, but which, though highly broadened, is still clearly seen in the fit for the AuNP data in part b. for the AuNP sample at this 30 Torr Bz loading, likely due to the formation of diamagnetic final states on the AuNP surfaces, 3,5 for which there is no clear route were MuC ̇6H 6 located at SiO 2 /SiOH sites. Were Bz to be located at silica sites in the AuNP/silica samples, little or no difference in ALC amplitudes would be expected, contrary to the observation in Figure 4.…”

“…1,2,4,19 However, these can change the nature and stability of metal NPs 20 and hence their surface reactivity. Recently, we explored hydrogen atom interactions with AuNPs utilizing the light hydrogenic H atom "muonium", composed of a positive muon and an electron (Mu = μ + e − , mass 0.114 amu 5,7,21−23 ), in studies of its surface magnetism 3,4 and reactivity on AuNPs, 3,5,23 encapsulated in mesoporous (SBA-15) silica hosts. 3,11,23−25 The porous silica not only prevents the aggregation of the AuNPs but importantly also provides for measurements on uncapped NP surfaces.…”