First observation of reversible mechanochromism and chromaticity study on calcium–copper–titanate

Abstract

First ever intriguing reversible mechanochromism espied in and chromaticity study on polycrystalline CaCu3Ti4O12 composition milled up to 16 hours are reported. The composition changes its color from blackish‐brown (un‐milled sample)‐reddish‐brown (1 hours milled sample)‐ heina‐green (6 hours milled sample)‐spinach‐green (16 hours milled sample) mainly due to the phase separation and corresponding change in band‐gap energy on milling. The UV‐Vis spectra show three observable absorption edges correspond to dx2-y2↔ditalicxy, dx2-y2↔dz2, and dx2-y2↔ditaliczx, dyz transitions. A marked mechanochromic effect with crystallite size reduction is explained in the light of the difference in the site group symmetry of CuO4 and ligand‐metal charge transfer coupled with Cu2+ d‐d transitions. The observed mechanochromism is reflected and well supported by photoluminescence spectral analysis. CIE chromaticity coordinates are located in the light blue region for an un‐milled sample and violet color region for 6 hours milled sample. The profound correlation between the structural and optical properties has been revealed.

Porous calcium copper titanate electrodes for paracetamol degradation by electro-oxidation via CuO-induced peroxymonosulfate activation

“…68 The change in the balance between TiO 6 and TiO 5 might explain the observed photoluminescence quenching. 68 All samples displayed similar spectral shapes, suggesting that all materials had a very similar electronic band structure. 40 In the samples with PMMA, the overall photoluminescence intensity decrease, without shifts in peak positions, suggests an increase in the concentration of specific surface defects in the CCTO material that might promote separation of the photogenerated charges.…”

Identification of structural changes in CaCu3Ti4O12 on high energy ball milling and their effect on photocatalytic performance

“…Comparable blue shift and relative intensity changes have been observed by P. Y. Raval et al on the milling of CaCu 3 Ti 4 O 12 , which were attributed to a lowering in the point symmetry of [CuO 4 ] units from D 2h (square planar coordination) to D 2 (highly distorted tetrahedral coordination), combined with a reduction in particle size. 21 Yet, no other evidence is provided to support such a change in Cu local coordination (note such as change is not supported from Cu XANES vide infra ). Hailili et al also saw significant changes in spectra for CaCu 3 Ti 4 O 12 prepared using different molten salt synthesis techniques.…”

“…Assignment of the observed bands has previously been attributed to simple electronic transitions of square planar Cu 2+ and to more complex transitions between band structures. 4,21,31 Specifically, the low energy feature (α) is attributed to d-d electronic transitions from occupied Cu(3d)-O(2p) states to the unoccupied localised α*Cu(3d)-O(2p) states within the band gap. Features (β) and (γ) can be attributed to electronic transitions from the valance band to the Ti (3d) conduction band.…”

“…Comparable blue shift and relative intensity changes Table 1 Refinement agreement factors, quantitative phase analysis, and unit-cell parameters with their standard deviations for the CCTO and CCTO-GM samples (prv and ten stands for, CaTiO 3 GdFeO 3 -type and CuO tenorite-type phases, respectively) (highly distorted tetrahedral coordination), combined with a reduction in particle size. 21 Yet, no other evidence is provided to support such a change in Cu local coordination (note such as change is not supported from Cu XANES vide infra). Hailili et al also saw significant changes in spectra for CaCu 3 Ti 4 O 12 prepared using different molten salt synthesis techniques.…”

“…Mechanical milling of CaCu 3 Ti 4 O 12 represents an appealing strategy to increase surface area substantially, retain phase purity (if the process is sufficiently controlled), and also induce structural defects. The latter point being highlighted in a series of papers by Modi and co-workers, 20,21 where on milling, positron annihilation spectroscopy and photoluminescence measurements confirmed an increase in structural defects, UV-vis spectroscopy that mechano-chromism can be induced, and lastly that antiferromagnetic properties are disrupted.…”