Distribution Pattern of Length, Length Uniformity, and Density of TiO₃²⁻ Quantum Wires in an ETS‐10 Crystal Revealed by Laser‐Scanning Confocal Polarized Micro‐Raman Spectroscopy

Abstract

ETS-10 is a highly intriguing microporous titanosilicate [1][2][3] that has shown an excellent propensity for the selective removal of harmful heavy-metal ions, [4][5][6][7][8][9] the potential to work as an effective catalyst for various reactions, [10][11][12][13][14][15] and that can be used as a material for solar cells.[ (Figure 1). [1][2][3][17][18][19][20][21][22][23][24][25][26][27] The TiO 3 2À quantum wire is a one-dimensional (1D) extreme of three-dimensional (3D) bulk titanates, which are widely used in industry as, for example, capacitors. [28] It also exhibits an interesting 1D quantum confinement effect. [17] The TiO 3 2À quantum wires are not expected to be connected all the way from one face to the opposite face of a crystal owing to the large number of randomly distributed defects. [17][18][19][20][21][22][29][30][31][32][33][34][35][36][37] Now the questions are what is the average length of the wires, to what degree do the lengths vary (how does the length homogeneity vary), how does the local density of the quantum wire vary from one region to another within a crystal, do they vary randomly or in accordance with a certain pattern? Answers to the above questions will be highly useful for understanding the mechanism of ETS-10 formation and growth, the refinement of its structure, improvements of its catalytic activities, and its future applications. However, there have been no methods to gain such information.The TiO 3 2À quantum wire in ETS-10 gives a strong Raman shift band between 724 and 840 cm À1 , arising from a longitudinal vibrational mode of the -Ti-O-Ti-O-chain. Its frequency at the band maximum (n max ), its bandwidth (full width at half maximum, fwhm), and intensity (I) reflect the relative average length, length homogeneity, and density of the quantum wire, respectively. The Raman band frequency decreases as the length increases, owing to the increase in the reduced mass of the quantum wire. The smallest frequency ever observed is 724 cm À1 . Bandwidths between 23 and 120 cm À1 have been observed, and the bandwidth decreases as the length uniformity increases. The intensity increases as the number of the TiO 3 2À quantum wire increases. Accordingly, the frequency, bandwidth, and intensity have served as the three important criteria for comparison of the relative average lengths, relative average length uniformities, and relative average densities of the TiO 3 2À quantum wires in the ETS-10 crystals. This information indicates that we can also apply the same principle to obtain their distribution pattern within an ETS-10 crystal if we can obtain a matrix of Raman spectra measured from a large number of artificially divided very small sections of a crystal. Furthermore, the obtained data would be more informative if we can obtain a map of these three data sets for the TiO 3 2À quantum wires running along the [110] and [11 0] directions, respectively.We now report that laser scanning confocal polarized micro-Raman (LSC-PMR) spectroscopy is a highly useful tool for the above purpose a...

Low-Overpotential Hydrogen Evolution Reaction Electrode Loaded with Very Small Amounts of PtPd Alloy Nanoparticles

“…Furthermore, it shows that fibers running perpendicular to the edges or the fibers running parallel to the [110] and [1–10] directions (brown wires in Figure d) mostly remain in the 2 h etched ETS-10. The above results coincide well with our optically derived previous results (Figure d) . Another TEM image of a partially etched Na/K-ETS-10 is shown in Figure e.…”

Predicting crystal growth via a unified kinetic three-dimensional partition model

“…This mimics almost exactly the optical microscopy images that show identical zoning and has been a source of debate for many years 23 . Similarly, in ETS-10 which displays rod growth ( Figure E6) rather than layer growth the incompleteness of the rods results in internal defects that congregate in a zone from the (001) facets to the centre of the crystal just as observed experimentally by Raman microscopy 24 . Our kinetic 3-D partitioning model shows that a straightforward growth mechanism can explain these optical phenomena without the need to resort to complex arguments related to twinning of the crystals.…”

Supramolecular Organization in Confined Nanospaces

“…Also, the titanosilicate ETS‐10 is a remarkable material, featuring a low‐dimensionality nanosystem encaged into a zeolite framework. Symmetrically distributed TiO 3 2− wires occupy zeolite nanochannels, running along perpendicular directions (Figure ) ,. Significantly, the wires displayed neat quantum size effects – band gap dependency on the wire length – and very promising optical properties ,…”

“…The quantum wires have different length and are arranged into a symmetric pattern. Reproduced by permission from Wiley‐VCH …”