Browsing by Author "Yamauchi, Y."

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Open Access
Continuous mesoporous pd films by electrochemical deposition in nonionic micellar solution
(American Chemical Society, 2017) Iqbal, M.; Li C.; Wood, K.; Jiang B.; Takei, T.; Dag, Ö.; Baba, D.; Nugraha, A. S.; Asahi, T.; Whitten, A. E.; Hossain, M. S. A.; Malgras, V.; Yamauchi, Y.
Mesoporous metals that combine catalytic activity and high surface area can provide more opportunities for electrochemical applications. Various synthetic methods, including hard and soft templating, have been developed to prepare mesoporous/nanoporous metals. Micelle assembly, typically involved in soft-templates, is flexible and convenient for such purposes. It is, however, difficult to control, and the ordering is significantly destroyed during the metal deposition process, which is detrimental when it comes to designing precisely mesostructured materials. In the present work, mesoporous Pd films were uniformly electrodeposited using a nonionic surfactant, triblock copolymer poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide), as a pore-directing agent. The interaction between micelles and metal precursors greatly influences the metal growth and determines the final structure. The water-coordinated species interact with the ethylene oxide moiety of the micelles to effectively drive the Pd(II) species toward the working electrode surface. From small-angle neutron scattering data, it is found that spherical P123 micelles, with an average diameter of ∼14 nm, are formed in the electrolyte, and the addition of Pd ions does not significantly modify their structure, which is the essence of the micelle assembly approach. The uniformly sized mesopores are formed over the entire mesoporous Pd film and have an average pore diameter of 10.9 nm. Cross-sectional observation of the film also shows mesopores spanning continuously from the bottom to the top of the film. The crystallinity, crystal phase, and electronic coordination state of the Pd film are also confirmed. Through this study, it is found that the optimized surfactant concentration and applied deposition potential are the key factors to govern the formation of homogeneous and well-distributed pores over the entire film. Interestingly, the as-prepared mesoporous Pd films exhibit superior electrocatalytic activity toward the ethanol oxidation reaction by fully utilizing the accessible active surface area. Our approach combines electrochemistry with colloidal and coordination chemistry and is widely applicable to other promising metals and alloy electrocatalysts.
Open Access
Electrochemical deposition of large-sized mesoporous nickel films using polymeric micelles
(Royal Society of Chemistry, 2018) Baba, D.; Kim, J.; Henzie, J.; Li, C.; Jiang, B.; Dağ, Ömer; Yamauchi, Y.; Asahi, T.
Stable mesoporous nickel (Ni) films can be prepared using polystyrene-b-poly-(oxyethylene) (PS-b-PEO) micelles as sacrificial templates. In this method, positively charged Ni precursors form hydrogen bonds with the PEO segments of the micelles, which are then co-electrodeposited on the surface of a working electrode. Changing the applied voltage during electrodeposition modifies the deposition rate and ultimately controls the architecture of the mesoporous Ni film.
Open Access
Electrochemical synthesis of mesoporous architectured Ru films using supramolecular templates
(Wiley-VCH Verlag, 2020) Kani, K.; Henzie, J.; Dağ, Ömer; Wood, K.; Iqbal, M.; Lim, H.; Jiang, B.; Salomon, C.; Rowan, A. E.; Hossain, M. S. A.; Na, J.; Yamauchi, Y.
The electrochemical synthesis of mesoporous ruthenium (Ru) films using sacrificial self‐assembled block polymer micelles templates, and its electrochemical surface oxidation to RuOx is described. Unlike standard methods such as thermal oxidation, the electrochemical oxidation method described here retains the mesoporous structure. Ru oxide materials serve as high‐performance supercapacitor electrodes due to their excellent pseudocapacitive behavior. The mesoporous architectured film shows superior specific capacitance (467 F g−1 Ru) versus a nonporous Ru/RuOx electrode (28 F g−1 Ru) that is prepared via the same method but omitting the pore‐directing polymer. Ultrahigh surface area materials will play an essential role in increasing the capacitance of this class of energy storage devices because the pseudocapacitive redox reaction occurs on the surface of electrodes.
Open Access
Electrochemical synthesis of mesoporous gold films toward mesospace-stimulated optical properties
(Nature Publishing Group, 2015) Li C.; Dag Ö.; Dao, T.D.; Nagao, T.; Sakamoto, Y.; Kimura, T.; Terasaki O.; Yamauchi, Y.
Mesoporous gold (Au) films with tunable pores are expected to provide fascinating optical properties stimulated by the mesospaces, but they have not been realized yet because of the difficulty of controlling the Au crystal growth. Here, we report a reliable soft-templating method to fabricate mesoporous Au films using stable micelles of diblock copolymers, with electrochemical deposition advantageous for precise control of Au crystal growth. Strong field enhancement takes place around the center of the uniform mesopores as well as on the walls between the pores, leading to the enhanced light scattering as well as surface-enhanced Raman scattering (SERS), which is understandable, for example, from Babinet principles applied for the reverse system of nanoparticle ensembles. © 2015 Macmillan Publishers Limited. All rights reserved.
Open Access
First synthesis of continuous mesoporous copper films with uniformly sized pores by electrochemical soft templating
(Wiley-VCH Verlag, 2016) Li C.; Jiang B.; Wang, Z.; Li Y.; Hossain, M. S. A.; Kim, J. H.; Takei, T.; Henzie, J.; Dag, Ö.; Bando, Y.; Yamauchi, Y.
Although mesoporous metals have been synthesized by electrochemical methods, the possible compositions have been limited to noble metals (e.g., palladium, platinum, gold) and their alloys. Herein we describe the first fabrication of continuously mesoporous Cu films using polymeric micelles as soft templates to control the growth of Cu under sophisticated electrochemical conditions. Uniformly sized mesopores are evenly distributed over the entire film, and the pore walls are composed of highly crystalized Cu.
Open Access
Lyotropic liquid crystalline mesophases made of salt-acid-surfactant systems for the synthesis of novel mesoporous lithium metal phosphates
(Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim, 2019) Uzunok, Işıl; Kim, J.; Çolak, Tuluhan O.; Kim, D.; Kim, H.; Kim, M.; Yamauchi, Y.; Dağ, Ömer
Mesoporous lithium metal phosphates are an important class of materials for the development of lithium ion batteries. However, there is a limited success in producing mesoporous lithium metal phosphates in the literature. Here, a lyotropic liquid crystalline (LLC) templating method was employed to synthesize the first examples of LiMPO4 (LMP) of Mn(II), Co(II), and Ni(II). A homogeneous aqueous solution of lithium and transition metal nitrate salts, phosphoric acid (PA), and surfactant (P123) can be spin coated or drop‐cast coated over glass slides to form the LLC mesophases which can be calcined into mesoporous amorphous LMPs (MA‐LMPs). The metal salts of Mn(II), Co(II) and Ni(II) produce MA‐LMPs that crystallize into olivine structures by heat treatment of the LLC mesophase. The Fe(II) compound undergoes air oxidation. Therefore, both Fe(II) and Fe(III) precursors produce a crystalline Li3Fe2(PO4)3 phase at over 400 °C. The MA‐LMPs show no reactivity towards lithium, however the crystalline iron compound exhibits electrochemical reactivity with lithium and a good electrochemical energy storage ability using a lithium‐ion battery test.
Open Access
Mesoporous metallic rhodium nanoparticles
(Nature Publishing Group, 2017) Jiang B.; Li C.; Dag, Ö.; Abe, H.; Takei, T.; Imai, T.; Hossain, M. S. A.; Islam, M. T.; Wood, K.; Henzie, J.; Yamauchi, Y.
Mesoporous noble metals are an emerging class of cutting-edge nanostructured catalysts due to their abundant exposed active sites and highly accessible surfaces. Although various noble metal (e.g. Pt, Pd and Au) structures have been synthesized by hard- and soft-templating methods, mesoporous rhodium (Rh) nanoparticles have never been generated via chemical reduction, in part due to the relatively high surface energy of rhodium (Rh) metal. Here we describe a simple, scalable route to generate mesoporous Rh by chemical reduction on polymeric micelle templates [poly(ethylene oxide)-b-poly(methyl methacrylate) (PEO-b-PMMA)]. The mesoporous Rh nanoparticles exhibited a ∼1/42.6 times enhancement for the electrocatalytic oxidation of methanol compared to commercially available Rh catalyst. Surprisingly, the high surface area mesoporous structure of the Rh catalyst was thermally stable up to 400 °C. The combination of high surface area and thermal stability also enables superior catalytic activity for the remediation of nitric oxide (NO) in lean-burn exhaust containing high concentrations of O 2.
Open Access
Molten salt assisted self assembly (MASA): synthesis of mesoporous metal titanate (CoTiO3, MnTiO3, and Li4Ti5O12) thin films and monoliths
(American Chemical Society, 2014) Avcı, C.; Aydın, A.; Tuna, Z.; Yavuz, Z.; Yamauchi, Y.; Suzuki, N.; Dag, Ö.
Mesoporous metal titanates are very important class of materials for clean energy applications, specifically transition metal titanates and lithium titanates. The molten salt assisted self-assembly (MASA) process offers a new synthetic route to produce mesoporous metal titanate thin films. The process is conducted as follows: first a clear solution that contains two solvents (namely the hydrated salt (Co(NO3)2· 6H2O or Mn(NO3)2·6H2O, or LiNO3·xH2O, and ethanol), two surfactants (cethyltrimethylammonium bromide, CTAB, and 10-lauryl ether, C12EO10), an acid and titanium source (titanium tetrabutoxide, TTB) is prepared and then spin or spray coated over a substrate to form a thin or thick lyotropic liquid crystalline (LLC) film, respectively. Finally, the films are converted into transparent spongy mesoporous metal titanates by a fast calcination step. Three mesoporous metal titanates (namely, CoTiO3, MnTiO3, and Li4Ti5O12) have been successfully synthesized and structurally/thermally characterized using microscopy, spectroscopy, diffraction, and thermal techniques. The mesoporous cobalt and manganese titanates are stable up to 500 °C and collapse at around 550 °C into nanocrystalline Co3O4− TiO2 and Mn2O3−TiO2; however, lithium titanate is stable up to 550 °C and crystalline even at 350 °C. The crystallinity and pore size of these titanates can be adjusted by simply controlling the annealing and/or calcination temperatures.
Open Access
Standing mesochannels: mesoporous PdCu films with vertically aligned mesochannels from nonionic micellar solutions
(American Chemical Society, 2018) Iqbal, M.; Kim, J.; Yuliarto, B.; Jiang B.; Li C.; Dağ, Ömer; Malgras, V.; Yamauchi, Y.
Mesoporous bimetallic palladium (Pd) alloy films with mesochannels perpendicularly aligned to the substrate are expected to show superior electrocatalytic activity and stability. The perpendicular mesochannels allow small molecules to efficiently access the active sites located not only at the surface but also within the film because of low diffusion resistance. When compared to pure Pd films, alloying with a secondary metal such as copper (Cu) is cost-effective and promotes resistance against adventitious poisoning through intermediate reactions known to impair the electrocatalytic performance. Here, we report the synthesis of mesoporous PdCu films by electrochemical deposition in nonionic micellar solutions. The mesoporous structures are vertically aligned on the substrate, and the final content of Pd and Cu can be adjusted by tuning the initial precursor molar ratio in the electrolyte solution.
Open Access
Symmetry-Breaking Plasmonic Mesoporous Gold Nanoparticles with Large Pores
(American Chemical Society, 2022-08-23) Nugraha, A. S.; Guselnikova, O.; Henzie, J.; Na, J.; Hossain, M. S. A.; Dag, Ömer; Rowan, A.; Yamauchi, Y.
Creating free-standing gold nanoparticles (Au NPs) with large pores is desirable because the exterior and interior voids can enhance electrocatalytic activity, mass transport, and optical extinction properties. However, the high mobility and significant positive reduction potential of Au precursors make it challenging to create Au NPs with pores of sufficient size to strongly interact with light. We demonstrate a method to synthesize mesoporous Au NPs with large, tunable pores. l-Cysteine acts as a metallogelator to form a dense, less mobile Au(I)-thiolate precursor that traps aggregated block copolymer micelles and facilitates the reduction of mesoporous Au NPs. Electron tomography measurements showed that the pores were distributed throughout the interior and exterior of the particle. Electrochemical methods were used to estimate the chemical reactivity of the surface active sites and estimate the accessible surface area of the pores to ensure that the metal surfaces were maximally accessible to the environment. The 3D models generated by tomography were then used to simulate their optical properties. Mesoporous Au NPs support multipolar plasmon resonances that penetrate deep into the interior pores of the NP. A simple model indicates that porosity affects the local optical conductivity of the NP by subdividing it into tiny nanoscale junctions that redshift the plasmon modes without changing the overall size or shape of the NPs. Large pores promote symmetry breaking, causing the quadrupolar and dipolar modes to overlap and form strongly hybridized plasmon modes. In the context of photocatalysis, porosity-induced symmetry breaking is advantageous because strong electric fields of the plasmon are colocalized along concave/convex features where step-edges and kinks in the atomic structure generate numerous catalytic active sites. Plasmon-enhanced photodegradation of metanil yellow was used to demonstrate the superior photocatalytic properties of meso Au NPs versus nonporous Au NPs.
Open Access
Two-dimensional mesoporous vanadium phosphate nanosheets through liquid crystal templating method toward supercapacitor application
(Elsevier, 2018) Mei, P.; Kaneti, Y. V.; Pramanik, M.; Takei, T.; Dağ, Ömer; Sugahara, Y.; Yamauchi, Y.
Mesoporous vanadium phosphate (VOPO4) nanosheets have been successfully synthesized through an easy and reproducible lyotropic liquid crystals (LLC) templating approach for the first time. Using the triblock copolymer (P123) as a surfactant, VOPO4 precursor with a well-developed 2D hexagonal mesostructure can be obtained. Following complete removal of the template by calcination, crystallized VOPO4 frameworks with less-ordered mesostructure are achieved. The as-prepared mesoporous VOPO4 nanosheets exhibit superior pseudocapacitive performance (767 F g‒1 at 0.5A g‒1) by virtue of the favorable mesostructure that gives rise to abundant easily accessible redox active sites as well as reinforced charge transfer and ion diffusion properties. The charge storage mechanism of the mesoporous VOPO4 nanosheets has been experimentally demonstrated to be based on the reversible two-step redox reactions between V(V) and V(III) in acidic medium. This advantageous LLC templating strategy is expected to open up a new route for designing various mesoporous metal phosphates with superior electrochemical performance for utilization in energy storage devices.