Electrolytic synthesis of aqueous aluminum nanoclusters and in situ characterization by femtosecond Raman spectroscopy and computations

Abstract

The selective synthesis and in situ characterization of aqueous Alcontaining clusters is a long-standing challenge. We report a newly developed integrated platform that combines (i) a selective, atomeconomical, step-economical, scalable synthesis of Al-containing nanoclusters in water via precision electrolysis with strict pH control and (ii) an improved femtosecond stimulated Raman spectroscopic method covering a broad spectral range of ca. T he importance of Al (aluminum) in the biosphere and to human civilization is enormous. The scale of mining and production of Al compounds is second only to that of Fe (iron). Our lives are influenced by its use in electronics (1, 2), cooking and eating utensils, and food packaging, and as structural materials in the construction, automotive, and aircraft industries. Its deposition and migration as a mineral ore are controlled by its aqueous chemistry and speciation. Millions of tons of Al compounds are used worldwide each year for water treatment, and it is found in all drinking water (3). The behavior of Al in water plays significant roles in soil chemistry and plant growth (4, 5), for example, governing Al bioavailability, toxicity, and its overall impact in aquatic ecosystems (6). Meanwhile, aqueous Al clusters are gaining importance as solution precursors for the large-area deposition of Al 2 O 3 coatings with broad technological applications (7,8).Despite more than a century of study (9, 10), the complete portrait of aqueous Al chemistry remains unclear. Studies of aqueous Al chemistry are notoriously difficult because of the variety and complexity of the species that can be formed, encompassing monomeric, oligomeric, and polymeric hydroxides (11-17); colloidal solutions and gels; and precipitates. Synthesis is complicated by the fact that the counter-ions and the method and rate of pH change all have dramatic effects on product formation (18,19). Few methods exist for the in situ determination and assignment of molecular-level structures. For instance, 27 Al NMR can only identify certain Al aqueous species (15). Furthermore, unlike organic compounds, systematic spectroscopic signatures of metal hydroxide clusters are less accessible, making interpretation of experimental spectra challenging. We hereby report a combined synthesis, experiment, and theory platform for the study of aqueous metal clusters. Electrolysis is exploited to control the solution pH and counter-ion content precisely during cluster synthesis without using chemical reagents. The evolution of solution species is followed in situ by an improved femtosecond stimulated Raman (FSR) technique (20-22) that can detect weak signals associated with structure-defining vibrational modes. The resulting pHdependent Raman spectra are interpreted by juxtaposition to quantum mechanically computed vibrational modes to assign specific molecular structures. Through this integrated approach, we have discovered a speciation behavior for Al in water that has not previously been observed. We focus here on the synthesis an...

Magnetic spin moment reduction in photoexcited ferromagnets through exchange interaction quenching: beyond the rigid band approximation

“…Using a femtosecond laser pulse to steer chemical reactions is the foundation of femtochemistry (Nobel prize in chemistry in 1999) [1]. This inspires the development of femtosecond Raman [2] and 2D IR spectroscopy [3]. A strong and ultrafast laser pulse can rip off and drive back electrons from gaseous atoms to generate high order harmonic generations, with emitted energy exceeding 200 eV and with time duration on the order of several hundred attoseconds (1 as=10 −18 s), representing an era of attophysics [4].…”

Sum-Frequency-Generation-Based Laser Sidebands for Tunable Femtosecond Raman Spectroscopy in the Ultraviolet

“…The large change in conjugation and electronic polarizability over the two-ring system leads to the observed strong Raman gain in comparison to other vibrational modes. Moreover, the amplification of the Stokes Raman spectrum primarily applies to the solute signal but not to spectral noise, so the experimental signal-to-noise ratio is greatly enhanced with the ~400 nm Raman pump and should be beneficial to characterize functional materials and molecular systems with intrinsically small electric polarizabilities [20,38].…”

Heat capacities and thermodynamics of formation of flat-Al13 nitrate – [Al13(OH)24(H2O)24](NO3)15·11H2O

“…The second main class contains clusters consisting solely of distorted octahedrally coordinated aluminum ions and hydroxyl linkages arranged analogously to those in a hexagonally close-packed layered hydroxide structure such as brucite [4]. These species increase in nuclearity and charge in a ring-like manner from [Al 8 (l 3 -OH) 3 (l 2 -OH) 12 (H 2 O) 18 ] 10+ (aq) [5] to [Al 13 (l 3 -OH) 6 (l-OH) 18 (H 2 O) 24 ] 15+ (aq) [6][7][8][9][10] (figure 1) and are termed ''flat'' structures, reflecting their more sheet-like topology compared to the Keggin Al 13 ions. These clusters have proven to be useful experimental models for understanding aqueous exchange reactions at the atomic level, as they expose similar (aluminum + oxygen) coordination chemistries found in soil minerals [11].…”

“…These clusters have proven to be useful experimental models for understanding aqueous exchange reactions at the atomic level, as they expose similar (aluminum + oxygen) coordination chemistries found in soil minerals [11]. Until recently the [Al 13 (l 3 -OH) 6 (l-OH) 18 (H 2 O) 24 ] 15+ (aq) (flat-Al 13 ) polynuclear aluminum hydroxide ions could not be synthesized in high yield or with fast reaction times [7,8,10]; thus they have often been omitted from speciation studies and relatively few reports are currently available on them, e.g. [4,[12][13][14].…”