A water-based plasma method creates stable multimetal nanoparticles with a protective shell that improves light driven carbon dioxide conversion, offering a practical path to efficient carbon transformation.
(Nanowerk Spotlight) Efforts to convert carbon dioxide into useful chemicals often run into the same core limitations. The molecule is stable, so it resists most attempts to activate it. Thermal routes consume large amounts of energy, while many light driven methods depend on ultraviolet wavelengths that supply only a small share of the solar spectrum.
Photothermal systems emerged as a response to these constraints. They use visible and infrared light to generate heat and energetic charge carriers at the same time. This pairing can reduce reaction barriers and support continuous operation without requiring precious metals. Progress in this area depends on materials that can manage several reaction steps, supply many active sites, and remain stable under light and heat.
High entropy alloys have attracted attention because they contain five or more metals in nearly equal amounts. Their mixed structure creates many types of local atomic sites. Earlier work on bulk alloys showed that this mixture can stabilize a single phase even when the component metals differ in size and chemistry.
At the nanoscale, the mixed arrangement supports surfaces with a wide range of binding environments. Studies on high entropy oxides and oxynitrides demonstrated that mixed cation systems can absorb visible light more effectively and guide carbon dioxide reduction with good selectivity. These results encouraged efforts to build non noble metal high entropy alloys for photothermal use.
However, synthesizing these alloys at the nanoscale remains difficult. Each metal behaves differently during heating, cooling, oxidation, and diffusion. Conventional methods often yield particles dominated by one metal or lead to phase separation as the system cools. Gas based routes require controlled atmospheres and often produce mixtures of phases. Rapid heating approaches and liquid metal templates help address some issues but depend on specialized conditions that limit scale.
The method uses a plasma formed between two alloy rods submerged in a water bath that also contains oxide support particles such as TiO₂. A plasma contains energetic electrons and ions. When it forms at the electrode tip, it melts small regions of the alloy surface. The resulting droplets enter the water, where they cool so quickly that the metals do not have time to separate. The oxide particles suspended in the water capture these droplets and stop them from merging.
The outcome is a set of nearly spherical nanoparticles roughly 200 nm in diameter anchored to the support. Chemical analysis shows that the particles preserve the intended mixture of iron, cobalt, nickel, chromium, and manganese. The study also demonstrates that the method can generate other high entropy compositions, which suggests broad versatility.
a) Schematic diagram of solution plasma to synthesize HEAs/TiO2, b) Comparison of conductivity and pH for different Rods after 1 h of discharge, HEAs-1 is FeCoNiCrMn, HEAs-2 is CoNiCuMoV, HEAs-3 is FeCoNiCrCu, HEAs-4 is FeCoNiCuMn, HEAs-5 is FeCoNiCuMo, HEAs-6 is Fe-CoNiCuV. (Image: Reproduced with permission from Wiley-VCH Verlag) (click on image to enlarge)
Microscopy and spectroscopy reveal how the particles organize themselves. Each particle contains a metallic core and an oxidized shell enriched in chromium and manganese. This arrangement forms because the five metals differ in how easily they oxidize.
Chromium and manganese form very stable oxides, meaning their oxides remain intact even when exposed to strong reducing conditions. Cobalt and nickel form less stable oxides that are easier for the energetic electrons and reactive hydrogen atoms in the plasma to reduce.
As the nanoparticles form, cobalt and nickel tend to stay metallic, while chromium and manganese concentrate at the surface in oxidized form. This surface layer thickens until it becomes compact and slows further oxidation. This behavior is known as self-limiting oxidation. It stabilizes the particle surface during synthesis and prepares a structure that can engage in several reaction steps.
The catalytic tests evaluate the alloy supported on TiO₂ using a mixture of carbon dioxide and hydrogen. The catalyst produces only carbon monoxide across the measured temperatures. Under illumination, the rate reaches 17.55 mmol g⁻¹ h⁻¹. When adjusted for the alloy loading of 5.87 wt.%, the activity becomes 298.1 mmol g_HEAs⁻¹ h⁻¹. This value is several times higher than that of single metal nanoparticles prepared with the same method.
Light also reduces the activation energy from 76.07 kJ mol⁻¹ in the dark to 44.81 kJ mol⁻¹ under illumination. Lower activation energy indicates that the reaction proceeds more easily. Control experiments confirm that the carbon monoxide originates from the supplied carbon dioxide. When the gas feed switches to nitrogen, carbon monoxide formation stops. When the experiment uses ¹³CO₂, the product consists entirely of ¹³CO.
Spectroscopic measurements reveal how the alloy works at the molecular level. Infrared data show that carbon dioxide attaches to the surface and forms intermediates such as carbonate and carboxylate groups. As temperature increases, the distribution of these groups changes. Bicarbonate signals weaken, while monodentate carbonate and carboxylate signals grow under illumination. These forms tend to break down more easily into carbon monoxide, which aligns with the increase in activity.
Light also shifts electron density within the alloy. X ray photoelectron data show that cobalt and nickel gain electrons when illuminated. Their binding energies shift to lower values. Chromium and manganese lose electrons and shift to higher values. Iron changes only slightly and lies between these behaviors.
An electron rich site is a site that takes up electrons under light, while an electron poor site is one that loses electrons. These shifts indicate that cobalt and nickel act as electron receiving sites, chromium and manganese act as electron donating sites, and iron transfers charge between the two groups.
Raman measurements add information about oxygen movement. As temperature rises in the reaction environment, signals linked to cobalt oxides weaken while those linked to chromium oxides strengthen. This suggests that oxygen formed during carbon dioxide reduction attaches first to cobalt or nickel and then moves to chromium or manganese. An oxygen vacancy is a missing oxygen atom within an oxide lattice. Filling these vacancies restores the surface and supports continued reaction.
Computational modeling supports these observations. The calculations examine the energies of the metals’ electrons relative to the Fermi level, the energy boundary between filled and empty electronic states. Metals with electron states below this level accept electrons more easily. In this alloy, cobalt and nickel fit this description. Chromium and manganese sit above the level and tend to lose electrons. Iron lies between these positions.
The modeling also evaluates how strongly carbon dioxide binds to each metal. The adsorption energy is negative when a molecule binds strongly. Carbon dioxide binds strongly to nickel with an energy of −0.997 eV and weakly to manganese with an energy of +3.102 eV. These findings support a mechanism in which chromium and manganese oxides split hydrogen into protons and electrons. The electrons move through iron toward cobalt and nickel sites. Carbon dioxide attaches to these electron rich sites, accepts electrons, and forms carbon monoxide and oxygen ions. The oxygen ions then move to chromium and manganese oxide regions, where they fill vacancies. This cycle keeps the surface active and prevents poisoning.
The study also evaluates the alloy on other oxide supports such as SiO₂, Al₂O₃, ZrO₂, and CeO₂. All supported catalysts produce only carbon monoxide. Activity increases with temperature and varies with the support. CeO₂ reaches the highest rate at 30.23 mmol g⁻¹ h⁻¹. The order reflects known differences in how easily oxygen moves through the supports. In all cases, illumination lowers the activation energy, showing that the alloy drives the photothermal effect regardless of support.
This study shows that solution plasma can form non noble metal high entropy alloy nanoparticles in water and that these particles can convert carbon dioxide under light and heat with high efficiency. The work links the structure of the particles to their performance and shows that similar behavior appears across several supports. These findings offer a strong foundation for developing multimetallic catalysts designed to use light and heat together to transform carbon dioxide.
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