A Unique Three-Dimensional Photocatalytic Structure Consisting of Highly Crystalline Na₂Ti₃O₇ Whiskers Grown from a NaCl Flux

Abstract

Highly crystalline and oriented Na 2 Ti 3 O 7 whiskers were successfully and directly grown on a metallic titanium mesh using a NaCl flux cooling method for application to semiconducting photocatalysis. Specifically, unique threedimensional photocatalytic materials were fabricated by converting a titanium mesh into a Na 2 Ti 3 O 7 tubular mesh consisting of highly oriented whiskers. The formation mechanism of the oriented Na 2 Ti 3 O 7 whiskers was investigated using field emission scanning electron microscopy, energy-dispersive X-ray spectrometry, and X-ray diffraction. From these results, the following conclusions could be confirmed. First, a titanium oxide layer was formed on the Ti wire surface by thermal oxidation. Since this oxide layer acts as a protective layer to thermal energy, the wire shape was maintained during Na 2 Ti 3 O 7 whisker growth. Next, highly oriented Na 2 Ti 3 O 7 whiskers were directly grown on the titanium oxide layer. During whisker growth, Ti was radially supplied from the bulk of the Ti mesh to the inner surface of the TiO 2 layer. Finally, the Ti wire structure transitioned into a tubular structure consisting of Na 2 Ti 3 O 7 whiskers. During the fabrication process, the Ti wire was dissolved by the NaCl flux, and the dissolved Ti ions reacted with O and Na ions to form TiO 2 and Na 2 Ti 3 O 7 layers. As a result, oriented and highly crystalline Na 2 Ti 3 O 7 whiskers were directly grown on the TiO 2 layer. The Ti wire was completely dissolved in this reaction system. Furthermore, the photocatalytic activities of the tubular mesh consisting of Na 2 Ti 3 O 7 whiskers were confirmed by the photocatalytic degradation of a fluoroalkylsilane self-assembled monolayer and trichloroethylene under ultraviolet light irradiation. The whisker mesh developed herein exhibited good photocatalytic properties under ultraviolet light irradiation.

Growth of ultralong potassium titanate whiskers by the KCl flux method with metallic titanium materials

“…[1][2][3][4][5][6][7][8] In addition, we recently reported on the flux growth of various highly crystalline functional whiskers, such as Na 2 Ti 6 O 13 , Na 2 Ti 3 O 7 , K 2 Nb 8 O 21 , CaMoO 4 , Ca 5 F(PO 4 ) 3 , Ca 5 Cl(PO 4 ) 3 and Ca 5 OH(PO 4 ) 3 . [9][10][11][12][13][14][15] Several types of potassium titanates have been reported, including K 2 Ti 2 O 5 and K 2 Ti 4 O 9 , which have layered structures, and K 2 Ti 6 O 13 and K 2 Ti 8 O 17 , which have tunnel structures. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] With their unique crystal structures, these potassium titanates have attracted attention for use in applications such as water splitting and degradation of toxic substances.…”

“…In our previous study, highly crystalline Na 2 Ti 3 O 7 whiskers were successfully and directly grown on a metallic titanium mesh by the NaCl flux method. 11 By applying this technique, we fabricated spherical structures consisting of potassium titanate whiskers in this study. Moreover, we demonstrated that the shape and surface morphology of the raw Ti materials affected the shapes and crystal phases of the synthesized whiskers.…”

Large Scale Preparation of β‐CaSiO3 Nanostructures by Solid‐State Reaction in NaCl–H2O(v) System at Lower Temperature

“…Although the exact mechanisms of the effect of NaCl and water vapor on the reaction between CaCO 3 and α‐SiO 2 are not very clear, the literatures reported that NaCl is one of the ionic crystals, which can offer a liquid condition for the growth of crystals over 801°C.In addition, the viscosity also affects the formation of crystals. NaCl ionic liquid has low viscosity, making the diffusivity of components become easier . Although the formation temperature of β‐CaSiO 3 in the reaction is below 801°C, it is well‐known that solid vibrational degrees of freedom at Tammann temperature (NaCl: about 440°C) can reach about 70% for that at the melting point.…”

“…NaCl ionic liquid has low viscosity, making the diffusivity of components become easier. 14,15 Although the formation temperature of b-CaSiO 3 in the reaction is below 801°C, it is well-known that solid vibrational degrees of freedom at Tammann temperature (NaCl: about 440°C) can reach about 70% for that at the melting point. So NaCl can enhance the diffusivity of starting materials and the rate of solid-state reactions, and promote the crystallization at 700°C.…”

Conformal Growth of Anodic Nanotubes for Dye-Sensitized Solar Cells: Part II. Nonplanar Electrode

“…For instance, titanium meshes or wires were functionalized with microporous TiO 2 films for photoelectrocatalysis, 15 sodium titanate nanotubes for bioapplication, 16 oriented Na 2 Ti 3 O 7 whiskers for photocatalysis, 17 TiO 2 nanowire arrays for photoelectrochemical application, 18 TiO 2 nanotube arrays for solid-phase microextraction, 19 water photoelectrolysis, 20 21 quantum dot photovoltaics, 22 23 drug delivery, 24 photocatalysis, 25 and supercapacitors. 26 Dyesensitized solar cells (DSCs) have also been constructed with various nonplanar photoanodes, such as TiO 2 nanoparticles on stainless steel meshes, 27 on carbon nanotube fibers, 28 or on titanium wires, 29 TiO 2 nanorod arrays on carbon fibers, 30 oriented ZnO nanowire arrays on iron wires, 31 ZnO nanosheet arrays on stainless steel meshes, 32 Lidong Sun received his Ph.D. degree in 2012 from Nanyang Technological University, Singapore.…”