Vis and NIR diffused reflectance study in disordered bismuth manganate ‒ lead titanate ceramics

: This work shows a correlation between light reflectance, absorption, and morphologies of series of (1- x ) BM– x PT, ( x = 0.0, 0.02, 0.04, 0.08, 0.12, 0.16, 0.24) ceramics composite. The (1- x ) BM– x PT showed features of a black mirror with a low optical energy gap. The measured Vis-NIR diffused reflectance enabled the calculation of the energy gap using the modified Kubelka-Munk function. The estimated energy gap was lower than 1.5 eV related to low reflectance in the Vis-NIR range. Moreover, obtained histograms of grains, using scanning electron microscope, enabled the correlation between grains size and amount of lead titanate. We deduced from the ceramics surface morphology that marked porosity also induced reflectivity of low magnitude. We correlated the magnitude of the energy gap with phases of the BM-PT composite and with the electrical conductivity activation energy reported in the literature. Our results findings opened prospect studied materials for optical applications.


Introduction
Perovskite ABO3 oxide materials attract attention for photo-catalytic and solar cells studies. The energy gap, Egap, which separates the energies of the valence band maximum and conduction band minimum, determines the material possibilities for such applications [1]. Electronic structure tuning is necessary for many materials to obtain suitable optical Egap demanded for optical applications because it ought to be ~1.5 eV or lower.
The optoelectrical properties of oxides are related to the energies of the valence and conduction band. However, ABO3 structures allow modifying their properties in accord with performed chemical and physical treatment. The ABO3 valence band is formed by oxygen O 2p states hybridized with states of metal 3d, 4d, and 5d placed in the B position. The 4d and 5d metal ions placed in B sublattice usually form a wide gap in the electronic structure. For instance, several perovskite titanates and niobates show insulator Egap larger than 3 eV. Therefore, ultraviolet, UV, light range only is effective for energy harvesting. Contrary, the metal ions from the 3d group provide narrow Egap lower than 1 eV. Moreover, 3d metal ions, e.g., Fe, Mn, Cr, and Co, form the hybridized states in the vicinity of the valence band. Therefore, the Egap decrease in the ABO3 electronic structure related to 4d and 5d ions sublattices can be obtained by suitable doping with 3d metal ions [2][3][4][5][6][7]. Moreover, oxygen vacancies, VO, states form shallow and deep levels within the Egap. However, narrow gap and intra-gap levels markedly affect electrical conductivity and losses [8].
There are several routes to modify the optical Egap of these oxide ABO3 materials. The substitution of the metal ions can lead either to solid solutions or to the formation of precipitates. Minor phases may appear when the solubility limit is overstepped, and/or density functional theory, calculated Egap changes from 3.18 to 3.03 eV, dependably on surface termination [25]. The FP-LAPW method calculation showed that absorption coefficient and energy gap decreased dependably on hydrostatic pressure [26].
BM crystal structure depends on growth conditions. Its perovskite meta-stable form, BiMnO3, was only obtained when synthesis was performed under high hydrostatic pressure [27] and also by using the hydrothermal method [28]. When BM ceramics was sintered at standard high-temperature ambient air conditions, a thermodynamically stable two-phase self-composite compound was obtained. Two phases in equilibrium: the cubic I23 with the Bi12MnO20 composition and the orthorhombic Pbam with the BiMn2O5 composition, were determined. The self-composite BM was an electrically non-polar compound, which exhibited antiferromagnetic order in a low-temperature range with TNéel  39 K [29].
BM ceramics studied herein can be treated as a self-composite consisting of two phases. BM can be treated as a self-composite consisting of two phases. The BiMn2O5 phase exhibits calculated Egap = 0.78 eV [30] and the sillenite Bi12MnO20 exhibits intrinsic Egap = 1.86 eV [31]. In case of BiMnO3 phase, Egap obtained from density of states (DOS) calculation was 0.33 eV, 0.3-0.4 eV, and ~1.1 eV [32][33][34]. BM exhibited semiconductor features and measured activation energy of electrical conductivity, which varied from Ea = 0.13 eV and 0.20 eV to 0.32 eV when temperature increased from 120 K to 770 K [35]. Electrical conductivity was attributed to the polaronic mechanism, and simultaneously high values of permittivity occurred. What is more, the electrical conductivity relaxations, showing changeover to properties dominated by glassy features, were assigned to the strain-stress effect [36][37][38][39].
It is worth noting that BM-PT composite sintering in ambient air at high temperature resulted in mutual migration of ions that induced transformation of phases when the Pb and Ti ions of PT component diffused into the BM compound. Simultaneously, Bi and Mn migration affected the PT component [40,41]. For instance, it would be noted that the Bi2O3 phase, iso-structural with Bi20TiO32, transformed from the Bi12MnO20 parent phase of the BM self-composite. The Egap of the Bi2O3 compound varied from ~2 to ~3 eV dependably on technology [42,43].
The high magnitude electrical conductivity, permittivity, dielectric losses, relaxation processes behavior, and shortening relaxation times effects caused by external hydrostatic pressure and the stress field generated around the replaced ions, also observed for 0.96 BM-0.04 PT and 0.88 BM-0.12 PT composite ceramics, was reported in our previous papers [40,41]. The multiple states of Mn 2+ /Mn 3+ /Mn 4+ ions, which occur in the BM self-composite, may facilitate light absorption. Hence, the variance of phases should be considered as a possible origin of Egap tuning.
According to the literature, the optical Egap of the BM-PT composite components should be detected in different ranges of photon spectrum. The Egap = 3.4 eV of ideal stoichiometric PT corresponds to the ultraviolet range. The oxygen-deficient PT shows Egap = 2.6-2.9 eV in Vis range, the Egap = 1.86 eV of Bi12MnO20 relates to Vis range, and the BiMn2O5 Egap = 0.78 eV to NIR range. The ideal PT features are beyond our study frames. Contrary, the Egap of BM, BM-PT, and defected PT is to be tested in the Vis and the NIR range. Therefore, NIR energy spectrum analysis would detect energy levels induced by crystal lattice defects and doping.
A series of (1-x) BM-x PT, (x = 0.0, 0.02, 0.04, 0.08, 0.12, 0.16, 0.24) ceramics was sintered [39,41,44]. In this work, we studied diffused reflectance in the Vis-NIR range to determine the PT content influence on the BM-PT composite absorption coefficient and optical Egap magnitude. It should be noted that the photovoltaic efficiency also depends on properties of surface layer exposed to the incident light. Hence, we determined ceramics' morphology using scanning electron microscopy. Correspondence between light reflectance, estimated absorption coefficient, Egap magnitude, ceramics' morphology, their correspondence to electrical features, and possibilities of application of a BM-PT composite material are discussed in this manuscript.

Preparation of composite ceramics
Series of (1-x) BM-x PT (x = 0, 0.02, 0.04, 0.08, 0.12, 0.16, 0.24) ceramics was produced via standard high temperature sintering. A two-stage procedure was performed. BM ceramics [38] and PT powder [22] were prepared separately at the first step. BM ceramics were produced using high-temperature sintering in the air (TS = 1170 K for 2 h), according to the nominal BiMnO3 formula [35]. They were graphite-like in color. The stoichiometric PT powder, which showed grains of sizes on submicron level (< 500nm), was obtained from the hydrothermal synthesis. It exhibited tetragonal P4mm symmetry [22]. The reference PT pellets were produced, and they were light yellow in color. Then the BM and PT powders were mixed in stoichiometric ratio, milled for 1 h, and pressed to pellets at 20 MPa. The ceramics were sintered at TS = 1170 K in ambient air for 1 h [40,41]. The sintered BM-PT ceramics were hard, porous, and graphite-like in color.

Vis-NIR reflectance
An USB-650 VIS-NIR Optic Spectrometer (OceanInsight, Inc., USA) equipped with a krypton light source lamp and OceanView 2.0 software was used. The Vis-NIR diffused reflectance, R, spectra of the samples were recorded in the wavelengths, , 400-1000 nm range at room temperature. The absorption coefficient was calculated from the reflectance spectra. The optical absorbance spectra were evaluated and presented using a Tauc plot of the modified Kubelka-Munk function to determine the optical band gap.

Surface morphology
A scanning microscope JSM-5410 equipped with an energy dispersion X-ray spectrometer (EDS with Si(Li) X-ray detector) was used to characterize the morphology and elemental content. The measuring chamber vacuum was 10 -4 -10 -5 Pa.

SEM morphology
The grains' morphology of series x BM-(1-x) PT ceramics was studied by using SEM. The ceramics showed a variety of grain shapes: cubes, hexagon base polyhedrons, and tiny irregular forms. The voids and porosity were observed in ceramics surfaces. Figure 1 shows secondary electron images obtained for the reference BM and PT ceramics. The BM map shows polyhedral-shaped hexagonal base grains with different sizes, varying predominantly in 0.2 -2.0 μm in transverse dimension and 0.3 -4.0 μm in longitudinal dimension, which is clearly visible in obtained polyhedrons grain size distribution ( Figure 1a). There is a low amount of grains with longitudinal size above 6.0 μm. The PT ceramics shows homogenous morphology (Figure 1b). However, the grains' size is markedly smaller, lower than 1.0 μm.  The studied herein ceramic composites exhibit many caverns in the fracture surfaces ( Figure 5). Such cavities are typical for the reference bismuth manganate ceramics [35]. Their origin would be associated with the technological process and attributed to chemical reactions. The occurrence of these cavities, where well-formed grains are visible, is one common feature of the BM-PT ceramics. The other common feature is porosity. We can distinguish well-shaped hexagon base polyhedrons grains, which are loosely attached each to other. Any clear tendency in size grain distribution in respect to PT content contribution was not observed for the BM-PT composite series. The majority of grains exhibited transverse size, which did not exceed 2.0 μm. The main amount of grains showed longitudinal size lower than 7.0 μm. The exception occurred for 0.96 BM-0.04 PT and 0.84 BM-0.16 PT composition, where longitudinal size reached dimension up to 12.0 μm. We attributed the occurrence of the small amount of grains of size below 1.0 μm to the presence of the PT compound in the ceramics composite that is clearly visible for ceramics with higher content of PT.

Vis-NIR optical features
The room temperature diffused reflectance, R, spectra were measured in 400-1000 nm range, which included Vis-NIR spectrum. For the graphical estimation of the Egap magnitude, the Vis-NIR spectra recorded in the R mode were transformed to Kubelka-Munk function, which is proportional to the extinction coefficient, α. Determination of Egap was conducted using the Tauc method and relation of incident photon energy, E = hν, with the modified Kubelka-Munk function: α·hν ≈ B(hν − Egap) n .
(2) We presumed the indirect allowed transitions in the BM-PT composite. Hence, α(hν) 1/n versus h plots, where exponent n = 2, were applied for analysis. The Egap value was obtained by extrapolating the straight-line segment to the intersection with the h axis, that is for α(hν) 1/2 = 0 [45]. We performed such extrapolation fit both for NIR and Vis range, where flat slopes could be distinguished. Such an approach was justified because of the multi-phase structure of studied herein composites.
We note that the PT, BiMn2O5, Bi12MnO20, and their derivative phases, which resulted from sintering, would bring in individual electronic structure contributions. In the case of composite, doped, defected, and multi-phase materials, the resulting spectra can include contributions from particular energy gaps. Such modifications may introduce intraband gap states that reflect in the reflectance an additional band. A spectrum of any mixture, including a doping-modified and two-phase semiconductor, is the linear combination of the spectra of both components; (α(h))eff = a·α1(h) + b·α2(h)), where a and b determine the components' concentrations contributions, while α1(hν) and α2(hν) are the absorption coefficients of the assumed two phases contribution. The determination of energy gaps related to particular phases causes a dilemma of accuracy Tauc equation reads: { [ a· α1(h) + b ·α2(h))] ·h ) } 1/2 = B(hν − Egap), (4) and direct application of the Tauc method brings in a systematic shift in the estimated Egap magnitude [46]. However, as the needed approximation, we decided to conduct the spectra analysis using Tauc method independently for the NIR and Vis range, respectively. Such an approach was justified by the occurrence of at least two phases, which properties would determine R magnitude in the NIR and the Vis range, respectively.
The diffused reflectance spectra of BM and PT reference ceramics are shown in Fig R() spectra obtained for (1-x) BM-x PT composites are shown in Figure 8. R magnitude decreased when the PT content increased, from ~50-60 % to ~25-30 % despite that the reference PT R() varied in 40-100 % range (compare Figures 6 and 8). The R() common feature was its magnitude increase when wavelength increased and more steep change in R magnitude for  > 900 nm. A more or less distinct broad hump occurred in ~550-800 nm range, it shifted toward NIR range and became negligible for high content of PT component. Graphical representation of modified Kubelka-Munk function allowed us to determine Egap magnitude dependence on the (1-x) BM-x PT ceramics composition (Figure 9). We discerned straight-line segments in the (αh) 1/2 vs. h plots, both in Vis and NIR range, because of the presumed manifestation of the multi-phase (1-x) BM-x PT ceramics features.
The Egap of the order of ~1 eV was estimated from the Vis spectrum range and it showed a tendency to fluctuate with PT content change. The Egap of ~0.13-0.36 eV estimated from the NIR range also fluctuated with PT content increase. The variation of the estimated gap energies versus the PT content, x, is shown in Figure 10.

Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 29 October 2021
The BM and BM-PT ceramics are multi-phase composites [35,38,47]. BM is the self-composite consisting of the major BiMn2O5 and the minor Bi12MnO20 phase. The BM-PT composite consists of phases, which structure changed because of mutual migration of ions when the ceramics were sintered from the reference BM and PT [40,41]. Moreover, local fluctuations in composition and the occurrence of oxygen deficiency cannot be excluded. The graphite-black color of these compounds indicated light absorbance in Vis and NIR range. Hence, Egap lower than ~1.5 eV was expected. Contrary, the reference PT powder was the one-phase compound. The light-yellow color of this material suggested that the actual Egap magnitude, originated from a defected PT structure, would be lower from Egap = 3.4 eV reported for the ideal stoichiometric transparent crystal [18,19].
Therefore, the measured R() spectra were considered as effective quantities, which included contributions from individual phases of the BM-PT system. It should be noted that when the PT content in the BM-PT ceramics was higher, the lower R magnitude occurred. The R magnitude decrease from ~50-60% to ~25-30 % indicated the active role of the PT component and marked influence porosity. A standard flat and smooth surface, like a glassy black mirror, usually offers reflectance on 40-50% level. Lower reflectance would be achieved in the case of a textured surface when light is scattered and absorbed more effectively [48]. It should be noticed that the SEM test exhibited porosity and concave voids in the BM-PT ceramics surfaces. Therefore, despite any uncontrolled and random factor which cannot be excluded a priori, we deduce that that the low reflectance of the BM-PT ceramics can relate to the rough surface features.
Grain size also is one of the factors, which affect the photo-voltaic properties. The correlation between the increasing grain size, within the sub-micro range, and Egap magnitude increase was reported for perovskite materials [49]. However, in our studies any marked tendency in the BM-PT grain size was not determined (see Figures 1-5). On the other hand, we note that the reference PT powder grain was much smaller than the BM-PT compounds grain (compare Figure 1b to Figures 1a and 2-5). Hence, we deduce that tiny grains of the PT powder introduced to the sintered BM-PT ceramics also would be responsible for this effect. Hence, we conclude that both the small grains of the PT component and the porous texture of BM-PT ceramics surface may facilitate scattering, decrease reflectance to ~25%, and enhance light absorption.
We applied the Tauc procedure independently within the NIR and the Vis range. We followed the idea that particular phases of a composite can independently contribute to the overall effective reflectance spectrum.
The estimated Egap(NIR) magnitude, which fluctuated on 1.000.10 eV level was obtained both for the BM and the BM-PT compounds. It correlates to the Egap = 0.78 eV calculated for the major BiMn2O5 orthorhombic Pbam phase of the BM composite [30]. The correspondence to Egap = 1.10 eV seems to less valid because of the other, monoclinic structure of BiMnO3 phase considered for this calculation [34]. The other Egap magnitude was determined for BM-PT composites in the Vis range. The estimated Egap(Vis) fluctuated in 0.13-0.36 eV range when the PT content increased, showing no clear tendency. Such narrow Egap corresponds to the measured activation energy of electrical conductivity at room temperature reported in literature: Ea = 0.20 eV [35], and 0.24 eV for BM [39] and 0.34-0.40eV for BM-PT compounds [40,41]. The correspondence to the Egap = 0.33 eV calculated for the stoichiometric BiMnO3 [32] and Egap = 0.45eV calculated for the oxygendeficient BiMnO3-x [34] should also be noticed.
In the case of reference PT, determination of three different Egap magnitudes were proposed. The Egap,1(Vis) = 1.5 eV and Egap,2(Vis) = 2.45 eV was estimated for the intermediate orange -cyan light and blue-light Vis range, respectively. These result correspond to the literature data, Egap =1.3-2.5 eV [22] and Egap = 2.6 eV [21], were reported for the defected PT crystals. The Egap(NIR) = 1.25 eV corresponds to the Ea = 1.3 eV of electrical conductivity measured for the pellets obtained from the same PT powders [22]. One also can notice coincidence to the obtained from calculation additional states lying 1.1 eV below the Fermi energy within energy gap, which were attributed to VO defects occurrence [20]. Hence, such the coincidence suggest that participation of PT originated Egap(NIR) in the effective R() spectra might be justified. Therefore, VO defects and other crystal lattice defects influence on the measured actual R() spectrum and the determined Egap magnitude cannot be excluded.
In short summary, BM-PT composite ceramics show features interesting for photovoltaic application: (1) BM-PT rough surfaces provide low reflectance and high absorption efficiency; (2) Estimated Egap magnitude, lower than 1.5 eV, corresponds to light absorption in NIR range; (3) Egap magnitude of the order of 0.2 eV correlates to activation energies determined for the electrical conductivity.

Conflicts of Interest:
The authors declare no conflict of interest.