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Anodic Ag/TiO2 nanotube array formation in NaOH/fluoride/ethylene glycol electrolyte as a photoanode for dye-sensitized solar cells

Nyein N, Tan WK, Kawamura G, Matsuda A, Lockman Z

Nanotechnology, 2016 Sep 02;27(35):355605.
PMID: 27456036 DOI: 10.1088/0957-4484/27/35/355605

Self-organized, 23 μm-thick anodic TiO2 nanotube (TNT) arrays were formed in sodium hydroxide/fluoride/ethylene glycol (EG) electrolyte at 60 V for 60 min. The presence of sodium hydroxide (NaOH) in the fluoride/EG electrolyte accelerates the formation of the TiO2 nanotube arrays. The anodic film was then decorated with silver nanoparticles (Ag NPs) by the photodeposition process and used as a photoanode in a rear-side-illuminated dye-sensitized solar cell. The Ag NPs decorated TNT arrays, with the former having diameters of 10-30 nm formed from 0.2 M of Ag-precursor solution and exhibiting the highest photoconversion efficiency (η) of 3.7% and a short-circuit current density of 12.2 mA cm(-2) compared to η = 3% and short-circuit current density of 9.1 mA cm(-2) for a sample without Ag NPs. The increase in η is thought to be due to the surface plasmon resonance and excess electrons from the nanoparticles.
Fulltext Mechanisms of removal of heavy metal ions by ZnO particles

Le AT, Pung SY, Sreekantan S, Matsuda A, Huynh DP

Heliyon, 2019 Apr;5(4):e01440.
PMID: 31008388 DOI: 10.1016/j.heliyon.2019.e01440

Effluent discharges from industry and domestic waste containing unknown inorganic pollutants. In this work, different mechanisms of heavy metal ions removal using ZnO particles were studied. ZnO particles were synthesized using solid precipitation technique. The morphology of ZnO particles was rod-like shape. The average length and diameter of ZnO particle were 497.34 ± 15.55 and 75.78 ± 10.39nm, respectively. These particles removed effectively heavy metal ions such as Cu(II), Ag(I) and Pb(II) ions with efficiency >85% under exposure of 1 hour of UV light. However, poor removal efficiency, i.e. <15% was observed for Cr(VI), Mn(II), Cd(II) and Ni(II) ions. The removal of these heavy metal ions was in the forms of metals or metal oxide via reduction/oxidation or adsorption mechanism.
Facile Fabrication of Plasmonic Enhanced Noble-Metal-Decorated ZnO Nanowire Arrays for Dye-Sensitized Solar Cells

Tan WK, Muto H, Ito T, Kawamura G, Lockman Z, Matsuda A

J Nanosci Nanotechnol, 2020 Jan 01;20(1):359-366.
PMID: 31383179 DOI: 10.1166/jnn.2020.17223

Novel decoration of high aspect ratio zinc oxide nanowires (ZnO NWs) with noble metals such as Ag and Au nanoparticles (NPs) was demonstrated in this work. A facile method of chemical deposition with good controllability, as well as good homogeneity would be a huge advantage towards large scale fabrication. The highlight of this work is the feasibility of multiple component decoration such as a hybrid (co-exist) Ag-Au NPs decorated ZnO NWs formation that could be beneficial towards the development of nanoarchitectured materials with the most desired properties. The local surface plasmon effect (LSPR) of Ag and Au NPs were confirmed using extinction spectra and significant photoelectrochemical conversion efficiency (PCE) enhancement of dye-sensitized solar cells (DSSCs) was achieved. The Ag-NPs and hybrid Ag-Au NPs decorated ZnO NWs marked an impressive 125 and 240% efficiency improvement against pure ZnO NWs. The improved dye light extinction resulted from the LSPR effect that had enabled greater electron generation leading to improved PCE. As the complex design of oxides' nanoarchitectures have reached a point of saturation, this novel method would enable further enhancement in their photoelectrochemical properties through decoration with noble metals via a simple chemical deposition route.
Nanoporous anodic Nb2O5 with pore-in-pore structure formation and its application for the photoreduction of Cr(VI)

Alias N, Hussain Z, Tan WK, Kawamura G, Muto H, Matsuda A, et al.

Chemosphere, 2021 Nov;283:131231.
PMID: 34144283 DOI: 10.1016/j.chemosphere.2021.131231

An anodic film with a nanoporous structure was formed by anodizing niobium at 60 V in fluorinated ethylene glycol (fluoride-EG). After 30 min of anodization, the anodic film exhibited a "pore-in-pore" structure; that is, there were smaller pores growing inside larger pores. The as-anodized film was weakly crystalline and became orthorhombic Nb2O5 after heat treatment. The energy band gap of the annealed nanoporous Nb2O5 film was 2.9 eV. A photocatalytic reduction experiment was performed on Cr(VI) under ultraviolet (UV) radiation by immersing the nanoporous Nb2O5 photocatalyst in a Cr(VI) solution at pH 2. The reduction process was observed to be very slow; hence, ethylenediaminetetraacetic acid (EDTA) was added as an organic hole scavenger, which resulted in 100% reduction after 45 min of irradiation. The photocatalytic reduction experiment was also performed under visible light, and findings showed that complete reduction achieved after 120 min of visible light exposure.
Photocatalytic performance of freestanding tetragonal zirconia nanotubes formed in H2O2/NH4F/ethylene glycol electrolyte by anodisation of zirconium

Rozana M, Soaid NI, Kian TW, Kawamura G, Matsuda A, Lockman Z

Nanotechnology, 2017 Apr 18;28(15):155604.
PMID: 28303803 DOI: 10.1088/1361-6528/aa5fac

ZrO2 nanotubes (ZrNTs) were produced by anodisation of zirconium foil in H2O2/NH4F/ethylene glycol electrolyte. The as-anodised foils were then soaked in the anodising electrolyte for 12 h. Soaking weakens the adherence of the anodic layer from the substrate resulting in freestanding ZrNTs (FS-ZrNTs). Moreover, the presence of H2O2 in the electrolyte also aids in weakening the adhesion of the film from the foil, as foil anodised in electrolyte without H2O2 has good film adherence. The as-anodised FS-ZrNTs film was amorphous and crystallised to predominantly tetragonal phase upon annealing at >300 °C. Annealing must, however, be done at <500 °C to avoid monoclinic ZrO2 formation and nanotubes disintegration. FS-ZrNTs annealed at 450 °C exhibited the highest photocatalytic ability to degrade methyl orange (MO), whereby 82% MO degradation was observed after 5 h, whereas FS-ZrNTs with a mixture of monoclinic and tetragonal degraded 70% of MO after 5 h.
Sunlight activated anodic freestanding ZrO2 nanotube arrays for Cr(VI) photoreduction

Bashirom N, Kian TW, Kawamura G, Matsuda A, Razak KA, Lockman Z

Nanotechnology, 2018 Sep 14;29(37):375701.
PMID: 29901455 DOI: 10.1088/1361-6528/aaccbd

Visible-light-active freestanding zirconia (ZrO2) nanotube (FSZNT) arrays were fabricated by a facile electrochemical anodization method in fluoride containing ethylene glycol electrolyte added to it was 1 vol% of potassium carbonate (K2CO3) at 60 V for 1 h. Poor adhesion at the metal∣oxide interface was induced by K2CO3 leading to the formation of FSZNT flakes. The effect of the crystal structures of the FSZNTs e.g., amorphous, amorphous/tetragonal, and tetragonal/monoclinic was investigated towards the photocatalytic reduction of 10 ppm hexavalent chromium, Cr(VI) at pH 2 under sunlight. The results demonstrate the amorphous FSZNTs exhibited the highest Cr(VI) removal efficiency than the crystalline FSZNTs (95% versus 33% after 5 h). The high photocatalytic activity of the amorphous FSZNTs can be attributed to enhanced Cr(VI) adsorption, high visible light absorption, and better charge carrier separation. The low photocatalytic activity of the crystalline FSZNTs annealed at 500 °C was mainly attributed to poor Cr(VI) adsorption, low visible light absorption, and less photoactive monoclinic-ZrO2.
Fulltext Formation of Dense and High-Aspect-Ratio Iron Oxide Nanowires by Water Vapor-Assisted Thermal Oxidation and Their Cr(VI) Adsorption Properties

Budiman F, Tan WK, Kawamura G, Muto H, Matsuda A, Abdul Razak K, et al.

ACS Omega, 2021 Oct 26;6(42):28203-28214.
PMID: 34723018 DOI: 10.1021/acsomega.1c04280

Coral-like and nanowire (NW) iron oxide nanostructures were produced at 700 and 800 °C, respectively, through thermal oxidation of iron foils in air- and water vapor-assisted conditions. Water vapor-assisted thermal oxidation at 800 °C for 2 h resulted in the formation of highly crystalline α-Fe2O3 NWs with good foil surface coverage, and we propose that their formation was due to a stress-driven surface diffusion mechanism. The Cr(VI) adsorption property of an aqueous solution on α-Fe2O3 NWs was also evaluated after a contact time of 90 min. The NWs had a removal efficiency of 97% in a 225 mg/L Cr(VI) solution (pH 2, 25 °C). The kinetic characteristic of the adsorption was fitted to a pseudo-second-order kinetic model, and isothermal studies indicated that the α-Fe2O3 NWs exhibited an adsorption capacity of 66.26 mg/g. We also investigated and postulated a mechanism of the Cr(VI) adsorption in an aqueous solution of α-Fe2O3 NWs.
Photoreduction of Cr(VI) in wastewater by anodic nanoporous Nb2O5 formed at high anodizing voltage and electrolyte temperature

Alias N, Hussain Z, Tan WK, Kawamura G, Muto H, Matsuda A, et al.

Environ Sci Pollut Res Int, 2022 Aug;29(40):60600-60615.
PMID: 35426025 DOI: 10.1007/s11356-022-20005-7

In this study, nanoporous anodic film was produced by anodization of niobium, Nb in a fluoride ethylene glycol electrolyte. The effect of anodization voltage and electrolyte temperature was studied to find an optimum condition for circular, ordered, and uniform pore formation. The diameter of the pores was found to be larger when the applied voltage was increased from 20 to 80 V. The as-anodized porous film was also observed to comprise of nanocrystallites which formed due to high field-induced crystallization. The nanocrystallites grew into orthorhombic Nb2O5 after post-annealing treatment. The Cr(VI) photoreduction property of both the as-anodized and annealed Nb2O5 samples obtained using an optimized condition (anodization voltage: 60 V, electrolyte temperature: 70 °C) was compared. Interestingly, the as-anodized Nb2O5 film was found to display better photoreduction of Cr(VI) than annealed Nb2O5. However, in terms of stability, the annealed Nb2O5 presented high photocatalytic efficiency for each cycle whereas the as-anodized Nb2O5 showed degradation in photocatalytic performance when used continually.
CdSe nanostructured thin film by electrophoretic deposition for quantum dots sensitized solar cell

Kyaw HMA, Ishak MN, Mohd Noor AF, Kawamura G, Matsuda A, Yaacob KA

Nanotechnology, 2024 Mar 18;35(23).
PMID: 38387094 DOI: 10.1088/1361-6528/ad2c5a

Cadmium selenide (CdSe) quantum dots (QDs) with different size, 2.5 and 3.2 nm, were successfully deposited on mesoporous titanium dioxide (TiO2) (Degussa-P25) nanostructures by electrophoretic deposition method (EPD) at the applied voltage 100 V for 120 s deposition time. In this study, the morphology of CdSe films deposited by EPD and the performance of the film when assembled into a solar cell were investigated. From the field emission scanning electron microscopy cross-section, the thickness of the CdSe nanoparticles with size 2.5 nm films were 3.4 and 3.0μm for CdSe 3.2 nm nanoparticles film. The structure of 2.5 nm is denser than compare of 3.2 nm CdSe nanoparticles. From UV visible spectroscopy, the band gap calculated for 2.5 nm CdSe nanoparticles is 2.28 eV and for 3.2 nm is 2.12 eV. Photovoltaic characterization was performed under an illumination of 100 mW cm-2. A photovoltaic conversion efficiency of 1.81% was obtained for 2.5 nm CdSe and 2.1% was obtained for 3.2 nm CdSe nanoparticles. This result shows that the photovoltaic efficiency is dependent on CdSe nanoparticle size.
Fulltext Ag nanoparticle-deposited TiO2 nanotube arrays for electrodes of Dye-sensitized solar cells

Kawamura G, Ohmi H, Tan WK, Lockman Z, Muto H, Matsuda A

Nanoscale Res Lett, 2015;10:219.
PMID: 26019696 DOI: 10.1186/s11671-015-0924-1

ABSTRACT: Dye-sensitized solar cells composed of a photoanode of Ag nanoparticle (NP)-deposited TiO2 nanotube (TNT) arrays were fabricated. The TNT arrays were prepared by anodizing Ti films on fluorine-doped tin oxide (FTO)-coated glass substrates. Efficient charge transportation through the ordered nanostructure of TNT arrays should be carried out compared to conventional particulate TiO2 electrodes. However, it has been a big challenge to grow TNT arrays on FTO glass substrates with the lengths needed for sufficient light-harvesting (tens of micrometers). In this work, we deposited Ag nanoparticles (NPs) on the wall of TNT arrays to enhance light-harvesting property. Dye-sensitized solar cells with these Ag NP-deposited TNT arrays yielded a higher power conversion efficiency (2.03 %) than those without Ag NPs (1.39 %).
PACS CODES: 06.60.Ei Sample preparation, 81.05.Bx Metals, Semimetals, Alloys, 81.07.De Nanotubes.
Formation of grassy TiO2 nanotube thin film by anodisation in peroxide electrolyte for Cr(VI) removal under ultraviolet radiation

Taib MAA, Alias N, Jaafar M, Razak KA, Tan WK, Shahbudin IP, et al.

Nanotechnology, 2020 Oct 23;31(43):435605.
PMID: 32640434 DOI: 10.1088/1361-6528/aba3d8

Arrays of TiO2 nanotubes (TiO2 NTs) with grassy surfaces were observed on titanium foil anodised at 60 V in fluorinated ethylene glycol (EG) with added hydrogen peroxide (H2O2). The grassy surface was generated by the chemical etching and dissolution of the surface of the TiO2 NTs walls, which was accelerated by the temperature increase on the addition of H2O2 . Upon annealing at 600 °C, the grassy part of the TiO2 NTs was found to consist of mostly anatase TiO2 whereas the bottom part of the anodic oxide comprised a mixture of anatase and rutile TiO2. The TiO2 NTs were then used to reduce hexavalent chromium (Cr(VI)) under ultraviolet radiation. They exhibited a rather efficient photocatalytic effect, with 100% removal of Cr(VI) after 30 min of irradiation. The fast removal of Cr(VI) was due to the anatase dominance at the grassy part of the TiO2 NTs as well as the higher surface area the structure may have. This work provides a novel insight into the photocatalytic reduction of Cr(VI) on grassy anatase TiO2 NTs.
Rapid Nucleation of Reduced Graphene Oxide-Supported Palladium Electrocatalysts for Methanol Oxidation Reaction

Ng JC, Tan CY, Ong BH, Matsuda A, Basirun WJ, Tan WK, et al.

J Nanosci Nanotechnol, 2019 Nov 01;19(11):7236-7243.
PMID: 31039881 DOI: 10.1166/jnn.2019.16717

Small sized electrocatalysts, which can be obtained by rapid nucleation and high supersaturation are imperative for outstanding methanol oxidation reaction (MOR). Conventional microwave synthesis processes of electrocatalysts include ultrasonication, stirring, pH adjustment, and microwave irradiation of the precursor mixture. Ethylene glycol (EG), which serves as a reductant and solvent was added during the ultrasonication or stirring stage. However, this step and pH adjustment resulted in unintended multi-stage gradual nucleation. In this study, the microwave reduction approach was used to induce rapid nucleation and high supersaturation in order to fabricate small-sized reduced graphene oxide-supported palladium (Pd/rGO) electrocatalysts via the delayed addition of EG, elimination of the pH adjustment step, addition of sodium carbonate (Na₂CO₃), prior microwave irradiation of the EG mixed with Na₂CO₃, and addition of room temperature precursor mixture. Besides its role as a second reducing agent, the addition of Na₂CO₃ was primarily intended to generate an alkaline condition, which is essential for the high-performance of electrocatalysts. Moreover, the microwave irradiation of the EG and Na₂CO₃ mixture generated highly reactive free radicals that facilitate rapid nucleation. Meanwhile, the room temperature precursor mixture increased supersaturation. Results showed improved electrochemically active surface area (78.97 m² g-1, 23.79% larger), MOR (434.49 mA mg-1, 37.96% higher) and stability.