Ru doping effect on the structural, electronic, transport, optical and dye degradation properties of layered Li₂MnO₃

Abstract

Ru doped Li₂MnO₃ compositions show the presence of redox couples Mn⁺³–Mn⁺⁴ and Ru⁺⁴–Ru⁺⁵. Due to the presence of these redox couples in Li₂Mn_1−xRu_xO₃ (x = 0.05, 0.1), Ru doped compositions show (i) change in structure (ii) large decrease in impedance(~ 10⁷ to ~ 10⁵ Ohm) (iii) degrade methyl orange and methylene blue solution (pH-6) in quick time in presence of tungsten (W) bulb and sun light. Synchrotron X-ray powder diffraction studies show the change in lattice parameters with Ru doping at Mn site in Li₂Mn_1−xRu_xO₃ (x = 0.05, 0.1). Raman and Infrared spectroscopic studies show the shift in Mn–O stretching mode of Li₂MnO₃ towards lower wave number with Ru doping. Soft X-ray absorption spectroscopic studies show the presence of mixed valences of Mn⁺³, Mn⁺⁴, Ru⁺⁴ and Ru⁺⁵ in Li₂Mn_1−xRu_xO₃ (x = 0.05, 0.1) using L_2,3 and M₄ edge. UV–Vis diffuse reflectance spectroscopic studies show the optical band gap in visible light range in Ru doped Li₂Mn_1−xRu_xO₃ (x = 0.05, 0.1) compositions. The presence of 5 mg of Li₂Mn_0.9Ru_0.1O₃ degrades 5 ml of methyl orange solution (10 mg/L) of pH 6 in 5 min and methylene blue solution (10 mg/L) of pH 6 in 3 min of exposure in W bulb light.

1 Introduction

Manganese (Mn) containing oxides crystallize in layered, perovskite, spinel, and various other structures, where valence state of Mn make these oxides vulnerable for various technological applications [1,2,3,4,5,6,7]. Colossal magnetoresistance, room temperature magnetism, metal to insulator transition, catalytic and Li intercalation are some of the properties which are utilized in devices [1, 2, 5, 6]. Mixed valence states of Mn play a major role in altering the properties of the materials as per the technological application need [2, 6, 7]. In perovskite oxides, ratio of Mn mixed valence Mn⁺³/Mn⁺⁴ decides the structural, magnetic and transport properties. Mn⁺³ containing oxides show insulating and antiferromagnetic behaviour while Mn⁺⁴ containing oxides also show insulating and antiferromagnetic properties [2, 6]. When Mn⁺³ remains present 67% and Mn⁺⁴ 33% then perovskite oxides show metallic and ferromagnetic behaviour. Presence of 50% Mn⁺³ and 50% Mn⁺⁴ valence states makes spinel oxides susceptible for charging and discharging in Li ion battery [1, 4, 7].

Hierarchical microstructure of Mn₃O₄ has been tried for degradation of methylene blue [8]. MnO₂, which has +4 valence state of Mn, has been considered as a photocatalyst and known for its cost effectiveness, adsorption/oxidizing abilities, large specific surface area and nontoxic nature [9]. It has been observed that some of the good photocatalysts involve metal cations Fe⁺² in photo-fanton process for degradation of MB, where redox interaction between Fe⁺² and Fe⁺³ play an important role to produce hydroxyl ions [10]. Ru is a 4 d transition metal and placed in same group with iron in periodic table. Ru-Mn interaction induces mixed valence Ru⁺⁴/Ru⁺⁵ with Mn⁺³/Mn⁺⁴ in perovskite oxides and spinel structures [11,12,13,14]. When Ru⁺⁴ (0.62 Å) is interact with Mn⁺⁴ (0.53 Å), it undergo redox reaction which leads to Mn⁺³ (0.645 Å) and Ru⁺⁵ (0.565 Å) valence states as redox potential of Ru⁺⁴/Ru⁺⁵ (1.07 eV) is comparable with that of Mn⁺³/Mn⁺⁴ (1.02 eV) [15]. Ru⁺⁴/Ru⁺⁵ redox couple operates at 4.3 V vs Li in the spinel LiMn₂O₄ and increases the cycling capacity of Li ion battery [12]. Ru doping brings cycling stability due to the presence of Ru⁺⁴/Ru⁺⁵ redox couple in Li₂Mn(Ru)O₃ rock salt type layered structure [16,17,18,19]. Presence of mixed valence Ru⁺⁴/Ru⁺⁵ with Mn⁺³/Mn⁺⁴ brings long range of magnetic ordering in La_0.7Ca_0.3Mn_1−xRu_xO₃ thin films [14]. Ru⁺⁴ containing thin film of La_0.7Ca_0.3Mn_0.7Ru_0.3O₃ shows colossal magnetoimpedance effects at room temperature [20]. Ru exists in its single valence state Ru⁺⁴ in LaMn_1−xRu_xO₃ (x = 0.1,0.2,0.3, 0.4) compositions and it shows band gad in the range of ~ 1.27 eV to 1.36 eV [21]. LaMn_1−xRu_xO₃ compounds degrade MO solution (pH 2.5) and generate photocatalytic oxygen by water in W bulb light [21]. In this study, we have documented the doping effect of Ru⁺⁴ at Mn⁺⁴ site in Li₂MnO₃ on the structure, Mn and Ru valence states, and dye degradation properties using methyl orange (MO) and methylene blue (MB) at nominal pH 6.

2 Experimental

Polycrystalline Li₂Mn_1−xRu_xO₃ (x = 0.0, 0.05, 0.1) samples were prepared using high temperature solid state reaction route. In the preparation process, we have weighed the stoichiometric amount of high purity LiOH.H₂O, MnO₂ and RuO₂. They were thoroughly mixed in an agate mortar and heated at 950 °C for 12 h in an ambient atmosphere in muffle furnace. Sr₄Ru₂O₉ sample was prepared by taking stoichiometric amount of high purity SrCO₃ and RuO₂ oxides. The precursors were thoroughly mixed in an agate mortar and calcined at 850 °C for 36 h in oxygen atmosphere, and subsequently sintered under oxygen atmosphere at 950 °C for 36 h in tubular furnace. LiMn₂O₄ sample was prepared by taking stoichiometric amount of high purity LiOH.H₂O and MnO₂. The precursors were thoroughly mixed in an agate mortar and heated at 800 °C for 24 h. LaMn_0.8Ru_0.2O₃ sample was prepared by taking stoichiometric amount of high purity La₂O₃, MnO₂ and RuO₂ oxides. The precursors were thoroughly mixed in an agate mortar and calcined at 900 °C for 24 h and subsequently sintered at 1050 °C for 24 h in ambient atmosphere. Heating rate was maintained 5 °C/min during heating and cooling. The X-ray powder diffraction patterns (step size 0.015, scan speed 1 s/step) using synchrotron radiation (λ = 1.09 Å) were recorded at room temperature at Indian beamline BL-18B, Photon Factory (PF), KEK, Tsukuba, Japan. The incoming X-ray beam from the bending magnet of the PF storage ring was collimated with a set of beam-defining slits having vertical opening of 0.2 mm and 2 mm in the horizontal direction. The sample was mounted onto an 8-circle goniometer (Huber, Germany) at the focal point of the focusing mirror of the beamline. A slit of 1.5 mm (horizontal) by 0.25 mm (vertical) was mounted just before the detector to increase the signal-to-background ratio. Raman spectra of these powder samples were recorded using Renishaw micro-Raman spectrometer (model RM-2000) and focused Ar⁺ laser beam of 514-nm using Leica microscope. The incident laser power was attenuated to 2 mW, and data acquisition time was set to 50 s for all the samples was used to record the data. The Raman spectra were recorded using Peltier air-cooled CCD detector in the range of 100–3200 cm⁻¹. Fourier transform infrared (FTIR) spectra (absorbance vs. wavenumber) of the samples were recorded using a FTIR Perkin Elmer spectrometer, using the KBr pellet technique from 400 to 4000 cm⁻¹ at room temperature. Each IR scan recorded after averaging of 25 scans. X-ray absorption spectroscopy (XAS) measurements for Mn L_2,3, and Ru M_4,5 were performed at the Soft X-ray absorption spectroscopy (SXAS) beamline (BL-01) of the INDUS 2 synchrotron source source (2.5 GeV, 300 mA) at Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, India. This beamline is operating in the energy range 100-1200 eV. Sample pellets were mounted in an ultrahigh vacuum (UHV) chamber with a base pressure of 10⁻¹⁰ Torr. SXAS data were collected in total electron yield (TEY) mode at room temperature under ultra-high vacuum conditions. Impedance and capacitance measurements were recorded using novacontrol make Impedance analyzer (Alpha-A high performance frequency analyzer) in the frequency range from 1 Hz to 40 MHz in ambient atmosphere at room temperature. We had used 10.4 mm diameter and 1.2 mm thick pellets polished both side with silver paste for these measurements. The wire length was ~ 2 feet between the impedance analyzer and sample. The reflectance of the Li₂Mn_1−xRu_xO₃ (x = 0.0, 0.05, 0.1) solid samples were recorded using UV–visible diffuse reflectance spectroscopy (DRS) in a Shimadzu UV-2450 spectrophotometer over a wavelength range of 200–800 nm. BaSO₄ was used as an internal reflectance standard. The absorbance of the Methyl Orange (MO) and Methylene Blue (MB) solution was measured in wavelength range of 200–800 nm to find out the degradation. To find out the band gap of these samples, the diffuse reflectance data were converted to the Kubelka–Munk function by equation [22]:

$${\text{F}}\left( {\text{R}} \right) = \left( {1 - {\text{R}}} \right)^{2} /2{\text{R}}$$

where R is reflectance, F (R) is proportional to extinction coefficient α.

Band gaps were determined by Tauc plots using equation [23]:

$$[\alpha {\text{h}}\upsilon \left] {^{{1/{\text{n}}}} = {\text{A }}} \right[{\text{h}}\upsilon - {\text{E}}_{\text{g}} ]$$

where υ is frequency, A is absorption constant, h is Planck constant and E_g is band gap. Here, n denotes the nature of band gap. For indirect band gap n = 2 and direct band gap n = 1/2 is used.

To understand the dye degradation ability of Li₂Mn_1−xRu_xO₃ (x = 0.0 and 0.1), we have selected methyl orange (MO) and methylene blue (MB) dyes for degradation. MnO₂ has been used as reference for MO and MB degradation. The photocatalytic degradation of aqueous methyl orange and methylene blue dyes by Li₂Mn_1−xRu_xO₃, and MnO₂, was carried out in a borosil make glass sample vials. Sample vial further placed in ultrasonicator and exposed in visible light of 200 Watt incandescent tungsten (W) bulb (300 to 900 nm) under constant flow of water (to maintain the constant temperature). MO and MB stock solutions of 10 mg/L concentration were prepared in ultra pure water. Hydrochloric acid was used to maintain pH of dye solutions. For dye degradation experiment, we have used 5 mg of catalyst in 5 ml dye solutions of 10 mg/L concentration for all the compositions. Before mixing catalysts in dye solutions, the pH of dye solutions was maintained 6. Ultrasonicator was used for proper mixing of catalyst and dye solution during exposing the mixture in visible light. The distance between 200 W incandescent tungsten(W) bulb and sample vial was maintained 15 cm. Visible part of sample vials (4 cm length and 1.6 cm diameter) was effectively exposed to 70.77 mW/cm² power of radiation. The samples were filtered using whatmann filter paper after exposing in W bulb light and absorbance of dye solutions were recorded in a Shimadzo make UV–Visible spectrometer over a wavelength range of 200–800 nm. The concentration of MB were calculated as C_t = C₀(A_t/A₀), where C₀ - initial MB concentration, A₀—initial absorbance (at λ = 662 nm), C_t—MB concentration at time t, and A_t—absorbance (at maxima of peak on t minute), respectively.The degradation rate  % was calculated by [(A₀ − A_t)/A₀] × 100.

3 Results and discussion

Li₂MnO₃ crystallizes in monoclinic unit cell and space group C2/m (Fig. 1). It is a layered structure, where a layer of Li ions, and a mixed layer of Li and 2Mn ions are alternating between closed packed oxygen layers. In a crystal, Li ions occupy three positions (2b, 2c and 4 h), oxygen ions occupy two positions (4i and 8j) and Mn atom occupies one position (4g) [24, 25]. In MnO₆ octahedra, two types of Mn–O bonds of 1.90 Å and 1.91 Å exists [24, 25].

Fig. 1

a Crystal structure of monoclinic Li₂MnO₃. b Shaded atoms (six O and one Mn atoms) show the configuration of Mn–O₆ octahedra in Li₂MnO₃

Figure 2 shows the synchrotron X-ray powder diffraction patterns of Li₂Mn_1−xRu_xO₃ [x = 0.0, 0.05, 0.1] compositions from 10 degree to 70 degree. We have found stoichiometric single phase for Li₂Mn_1−xRu_xO₃ [x = 0.0, 0.05, 0.1] compositions and diffraction patterns matches with the reference code ICSD-202639 and JCPDS-01-084-1634 patterns. All the peaks are matching with the reported results in literature. Parent and Ru doped diffraction patterns show the characteristic peaks (020), (110), (021) and (110) which confirms the crystallization of these compositions in C2/m. Figure 3 shows the close look of the peaks of normalized intensity for all the compositions. It has been observed that the peaks (001), (020), (110), (130) shifts at lower theta with the increase in Ru concentration. Shifting of peaks shows the expansion of unit cell with Ru substitution due to the higher ionic size of Ru⁺⁴ (0.76Å) than the ionic size of Mn⁺⁴ (0.67Å). Peaks (202) and (131) merge together with Ru substitution at Mn site. Peak ($\bar{3}$31) shifted towards lower theta value for Ru = 0.05 composition but for Ru-0.1 composition ($\bar{3}$31) peak split into ($\bar{3}$31) and (060). Crystallite size for all the composition has been calculated using Debye Sherrer formula (Crystallite size (Å) = 0.9 λ/d cosθ). Ru doping decreases crystallite size of materials. Table 1 summarizes the results of the analysis of synchrotron X-ray powder diffraction patterns. These results show that Ru has been successfully doped at Mn site in Li₂Mn_1−xRu_xO₃ [x = 0.05, 0.1]. To explore the shifting of peaks at lower theta value and lowering of crystallite size at micro level by Ru doping at Mn site, we have recorded the Raman spectra of these compositions [12, 26].

Fig. 2

Synchrotron X-ray powder diffraction patterns of Li₂Mn_1−xRu_xO₃ (x = 0.0, 0.05, 0.1). Ru doped compositions show the single phase as Li₂MnO₃. Peaks are indexed for Li₂MnO₃

Fig. 3

Zoomed peaks of synchrotron X-ray powder diffraction patterns of Li₂Mn_1−xRu_xO₃ (x = 0.0, 0.05, 0.1). Shifting of peaks towards lower theta value shows the expansion of lattice with Ru substitution at Mn in Li₂MnO₃

Table 1 Synchrotron X-ray powder diffraction data acquisition conditions and lattice parameters of Li₂Mn_1−xRu_xO₃

Figure 4 shows the Raman peaks for powder samples of Li₂Mn_1−xRu_xO₃ [x = 0.0, 0.05, 0.1] compositions. Theoretical calculations predict the 15 Raman active optical modes, in which 7 modes are A_g and 8 modes are B_g [27]. In monoclinic space group C2/m, A_g modes are generated from the symmetric vibrations of cations along b-axis. B_g modes are generated from the symmetric vibrations of cations along a and c-axis. The vibrations of octahedrally coordinated Mn atom at 4 g position results 1 A_g and 2B_g modes, while 1 A_g and 2B_g modes are due to Li atom at 4 h site. O atom at 4i position generates 2A_g and 1B_g modes, and remaining 3 A_g and 3B_g modes can be assigned to vibrations of O atom at 8j site [28, 29]. We have clearly observed 14 active Raman modes in our synthesized Li₂Mn_1−xRu_xO₃ samples while other groups have reported only 8 Raman peaks in monoclinic Li₂MnO₃ samples [28, 29]. The observed Raman Peaks positions are summarized in Table 2 for Li₂Mn_1−xRu_xO₃ [x = 0.0, 0.05, 0.1] compositions. Ru substitution at Mn site shifts A_g peak at ~ 615 cm⁻¹ of Li₂MnO₃ to lower value ~ 610.8 cm⁻¹ for x = 0.05 composition. This observation is similar to the Ru doped LiMn₂O₄ samples where we had observed the peak shifting towards lower wave numbers [12]. Ru as a 4d transition metal having extended orbitals and higher spin orbit coupling constant 800 cm⁻¹ in comparison with 100 cm⁻¹ of Mn. It makes Ru–O bond stronger than Mn–O bond which reflect in shifting of A_g vibrational peak towards lower wave numbers.

Fig. 4

Raman shift for powder samples of Li₂Mn_1−xRu_xO₃ (x = 0.0, 0.05, 0.1). Vertical lines show the peak positions for Li₂MnO₃

Table 2 Frequencies of experimentally observed Raman active modes of Li₂Mn_1−xRu_xO₃

For Ru = 0.1 composition, we have observed that Raman peaks splitted into multiple peaks for stretching modes of Mn–O vibrations, it shows that the multiple cations vibration contributing to stretching modes. Figure 5 shows the Infra red (IR) peaks for powder samples of Li₂Mn_1−xRu_xO₃ [x = 0.0, 0.1] compositions. Theoretical calculations predict the 7 A_u and 11 B_u IR active optical modes [27]. We have found 7 peaks for Li₂MnO₃ which are similar with the reported experimental studies in literature [30]. In Li₂Mn_0.9Ru_0.1O₃, some Raman peaks merge and total 5 peaks have been observed. All the peaks have been sihifted towards lower wavenumber with the doping of Ru at Mn site. Presence of multiple peaks and shift of other vibrational peaks in Raman and IR spectra motivated us to find the valence states of transition metals Mn and Ru in Li₂Mn_1−xRu_xO₃ [x = 0.05, 0.1] compositions.

Fig. 5

Peaks of Infrared spectra for powder samples of Li₂Mn_1−xRu_xO₃ (x = 0.0, 0.1)

In Figure 6a, we show Mn- L_2,3 absorption edge spectra of Li₂Mn_1−xRu_xO₃ [x = 0.0, 0.05, 0.1] and Li₂Mn₂O₄ systems together with that of Mn₂O₃, MnO₂ (where Mn exist in +3 and +4 valence states) for comparison. The L₂ edge and L₃ edge correspond to the 2p_1/2 − 3d (t_2g and e_g orbitals) and 2p_3/2 − 3d (t_2g and e_g orbitals) transitions, respectively. Cathode material LiMn₂O₄ is known for showing 50–50% of Mn⁺³ and Mn⁺⁴ valence states [12]. In L₃ absorption edge, We have observed two peaks at 642.30 eV and 643.39 eV in LiMn₂O₄ which are identical to peaks at 642.37 in Mn₂O₃ and 643.45 eV in MnO₂ [31]. It shows the presence of Mn⁺³ and Mn⁺⁴ valence states in Li₂Mn₂O₄ [31]. In Li₂MnO₃ (x = 0.0), we have found peak at 643.45 eV which is comparable to peak observed in MnO₂, it shows the presence of Mn⁺⁴ valence state in Li₂MnO₃. Figure 6b shows the close look of L₃ edge for Li₂Mn_1−xRu_xO₃ [x = 0.0, 0.05, 0.1]. In Li₂Mn_0.95Ru_0.05O₃ (x = 0.05), we have observed peaks at 643.35 eV and in Li₂Mn_0.9Ru_0.1O₃ (x = 0.1) we have found peaks at 643.25 eV. These peaks are shifting towards lower energy as Ru concentration is increasing. It confirms the presence of Mn⁺³ valence state along with Mn⁺⁴ in Li₂Mn_0.95Ru_0.05O₃ and Li₂Mn_0.9Ru_0.1O₃.

Fig. 6

a XAS spectra of L_2,3 edge for Li₂Mn_1−xRu_xO₃ [x = 0.0, 0.05, 0.1] and LiMn₂O₄. XAS spectra of MnO₂ and Mn₂O₃ used for reference of Mn⁺³ and Mn⁺⁴ valence states peaks. b L₃ edge of Ru doped Li₂MnO₃ compositions, which show shift in Mn peaks with Ru substitution

In Figure 7a, we show Ru – M₄ absorption edge spectra of Li₂Mn_1−xRu_xO₃ [x = 0.05, 0.1] and LaMn_0.8Ru_0.2O₃ systems together with that of RuO₂ and Sr₄Ru₂O₉, where Ru exist in +4 and +5 valence states [11, 15, 32, 33]. The M₄ edge and M₅ edge correspond to the electronic transition from Ru-3d_5/2 to Ru p orbitals and Ru-3d_3/2 to Ru p orbitals respectively. The M₄ edge of RuO₂ has been found at 287.95 eV which is similar with the reported by zhou et al. [33]. Zhou et al. have found M₄ edge of Ru at 288 eV in RuO₂ nano particles [33]. The M₄ absorption edge has been found at 289.74 eV for Sr₄Ru₂O₉, which is 1.78 eV more than M₄ edge peak of RuO₂. This result shows the presence of +5 valence state of Ru in Sr₄Ru₂O₉. These results are similar with findings of Hu et al. [32] where they have shown that Ru remains in +4 and +5 valence states for RuO₂ and Sr₄Ru₂O₉ respectively [32, 34]. In our observation, LaMn_0.8Ru_0.2O₃ has been found showing M₄ absorption peak of Ru at 288.05 eV which is very close to the absorption peak (at 287.95 eV) of RuO₂ [33]. This result is very much similar with the reported result by patra et al., where it has been found that Ru remains in +4 valence state in LaMn_0.8Ru_0.2O₃ [21]. In Li₂Mn_0.95Ru_0.05O₃ (x = 0.05), we have observed multiple peaks at 288.15 and 289.88 eV and similarly in (x = 0.1) two peaks at 288.02 and 289.88 eV have been observed. These two absorption peaks are similar with the M₄ absorption peak of RuO₂ and Sr₄Ru₂O₉. These results show that Li₂Mn_0.9Ru_0.1O₃ and Li₂Mn_0.95Ru_0.05O₃ contain multi valence states of Ru (Ru⁺⁴ and Ru⁺⁵). Fitting of M₄ absorption peak of Ru shows the % amount of Ru valence state in Li₂Mn_0.9Ru_0.1O₃ and Li₂Mn_0.95Ru_0.05O₃ (Fig. 7b). Li₂Mn_0.95Ru_0.05O₃ compound contains 48% Ru⁺⁴ valence state and 52% Ru⁺⁵ valence state. Li₂Mn_0.9Ru_0.1O₃ compound contains 32% Ru⁺⁴ valence state and 68% Ru⁺⁵ valence state.

Fig. 7

a XAS spectra of M₄ edge for Li₂Mn_1−xRu_xO₃ [x = 0.05, 0.1]. XAS spectra of RuO₂ and Sr₄Ru₂O₉ used for reference of Ru⁺⁴ and Ru⁺⁵ valence states peaks. b Fitting of Ru—XAS spectra shows the percentage of Ru⁺⁴ and Ru⁺⁵ valence states. Circles show the experimental data and red line shows the final peak sum of fit on experimental data

To see the effect of multivalence states of Ru and Mn in Li₂Mn_1−xRu_xO₃ [x = 0.05, 0.1] on the transport properties of Li₂MnO₃, we have measured the frequency dependent impedance measurements. Figure 8 shows the impedance and capacitance for these compositions in the frequency range 10³ to 4 × 10⁷ cm⁻¹. At frequency 10³ Hertz, x = 0.0 composition show 1.38 × 10⁷ Ohm impedance while x = 0.05 and 0.1 show 2.82 × 10⁵ and 5.37 × 10⁴ Ohm respectively. Capacitance is found 1.05 × 10⁻¹¹ F at frequency 10³ Hertz for x = 0.0 composition while x = 0.05 and 0.1 compositions show 5.15 × 10⁻¹⁰ F and 2.71 × 10⁻⁹ F respectively. These measurements show the decrease in impedance and increase in capacitance with the substitution of Ru in Li₂MnO₃. On increasing frequency, impedance further decreases for all the compositions. These observations confirm the presence of Mn⁺³ valence state along with Mn⁺⁴ valence state in Ru doped compositions. While synthesis of Li₂Mn_1−xRu_xO₃ [x = 0.0, 0.05, 0.1] compositions, we have observed that parent x = 0.0 composition is in reddish colour and Ru doped compositions x = 0.05 and 0.1 are in black colour. To find out the absorption capability in visible light, we have measured UV–vis diffuse reflectance spectroscopic measurements.

Fig. 8

a Impedance b capacitance versus frequency plots for Li₂Mn_1−xRu_xO₃ [x = 0.0, 0.05, 0.1] compositions

Figure 9 shows the reflectance of the Li₂Mn_1−xRu_xO₃ [x = 0.0, 0.05, 0.1] compositions. TiO₂ (commercial sample) is used as reference for reflectance. Optical reflectance spectra of Li₂MnO₃ showed a sharp edge at 610 to 750 nm, which is similar with the reported by Tamilarasan et al. [35]. The peaks ~ 610 nm corresponds to a HOMO–LUMO gap between filled O(2p) states and empty Mn(IV) (3d–4 s) states. Figure 10 shows the band gap of the Li₂Mn_1−xRu_xO₃ [x = 0.0, 0.05, 0.1] compositions, which was calculated using Kubelka–Munk function and Tauc plot [22, 23]. Tauc plot gave an energy band gap 2.17 eV for parent Li₂MnO₃ which is in agreement with the theoretical estimations [35, 36]. In Ru doped compositions, we have observed some more reflectance peaks near to 480 nm and 577 nm. Band gap calculation suggests the presence of multiple absorption of band gap 1.97, 2.15, 2.33 and 2.41 eV for x = 0.1 composition and band gap 1.91, 2.09 and 2.24 eV for x = 0.05 composition. These experimental determined optical band gaps are reported by theoretical studies on the systems which have Mn mixed valences.

Fig. 9

UV–vis (DRS) spectra for Li₂Mn_1−xRu_xO₃ [x = 0.0, 0.05, 0.1] compositions. TiO₂ used as a reference

Fig. 10

Band gap calculations using UV–vis (DRS) spectra for a F(R) for all the compositions b x = 0.0, c x = 0.05 d x = 0.1 in Li₂Mn_1−xRu_xO₃ [x = 0.0, 0.05, 0.1] compositions

The optical absorption in Mn containing systems is controlled by electric dipole matrix elements that preserve spins of electrons. Features observed in optical conductivity close to 1 eV, 3 eV have been ascribed due to the d–d charge transfer between Mn ions on different sites, e_g−e_g transitions in perovskite manganite systems [37,38,39]. These band gap values 1.91, 2.09 and 2.24 eV for x = 0.05 composition and band gap 1.97, 2.15, 2.33 and 2.41 eV for x = 0.1 composition suggest the optical activity of Li₂Mn_1−xRu_xO₃ [x = 0.05, 0.1] compositions in visible light.

Ranjeh et al. have investigated photocatalytic properties of Li₂MnO₃ bulk and nano particles using aqueous solution via visible light for degradation of dyes acid red88 and malachite green [40]. Acid red dye degradation was found 17.6% by bulk Li₂MnO₃ and 40.9% by nano Li₂MnO₃. They have shown following mechanism of degradation using Li₂MnO₃: [40]

$${\text{Li}}_{2} {\text{MnO}}_{3} + {\text{h}}\upsilon {\to} \left( {{\text{Li}}_{2} {\text{MnO}}_{3} } \right)* + {\text{e}}^{ - } + {\text{h}}^{ + }$$

$${\text{O}}_{2} + {\text{e}}^{ - } {\to}\,\, {\text{O}}_{2}^{ - }$$

$${\text{O}}_{2}^{ - } + \, 2{\text{H}}^{ + } {\to}\, {\text{H}}_{2} {\text{O}}_{2}$$

$${\text{OH}}^{ - } + {\text{ h}}^{ + } {\to}\, {}^{ \cdot }{\text{OH}}$$

$${\text{H}}_{2} {\text{O}} + {\text{h}}^{ + } {\to}\, {}^{ \cdot }{\text{OH}} + {\text{H}}^{ + }$$

$${\text{Dyes}}\ + \, \left( {{\text{O}}_{2}^{ - } /{}^{ \cdot }{\text{OH }}/{\text{H}}_{2} {\text{O}}_{2} } \right) {\to}\, {\text{ Degraded}}\;{\text{water}}\;{\text{pollutant}}$$

It has been shown that cationic additive enhances the degradation capability of Li₂MnO₃. These cations make positive surface of Li₂MnO₃ and prevent electron–hole recombination which help generation of.OH on the Li₂MnO₃ surface [40]. We have selected two different type of dyes, (i) A cationic dye, Methylene Blue (MB) and (ii) an azo dye methyl orange (MO) for degradation capabilities of Li₂MnO₃ and its Ru doped compositions. MB is used for printing, leather and in dyeing cotton and, causes various harmful effects such as eye burns, irritation to the skin and gastrointestinal tract [41]. MB is also regarded as significant threat due to its carcinogenic and mutagenic properties [42]. MB containing waste water from industry is essentially needed an efficient treatment technology which give ultimately clean water for safe disposal quickly. Methyl orange is used as an indicator for acid base titration.

In Figure 11, we have plotted the absorption in the wavelength range 200–800 nm of (i) as prepared MO solution (ii) pH controlled (pH = 6) MO solution and (iii) MO solution of pH6 mixed with Li₂Mn_0.9Ru_0.1O₃ composition exposed up to 5 min in visible light. Methyl orange absorbs UV–vis light at ~ 270 nm and ~ 465 nm which corresponds to n-π* transition. MO solution shows absorption peak ~ 465 nm which shifts to ~ 507 nm after adjusting the pH 6 of the MO solution. After 5 min of ultrasonication and exposure in visible light, MO solution with pH 6 is degraded and no peaks at ~ 465 and ~ 507 nm is visible. Li₂Mn_0.9Ru_0.1O₃ composition degrades MO solution 100% in 5 min in the presence of nominal pH6. As prepared MO solution show no degradation either in sunlight or with ultrasonication (Table 3).

Fig. 11

Absorbance spectra of the Methyl Orange solutions after reaction with Li₂Mn_0.9Ru_0.1O₃ in the given exposure time of W bulb light. Please note disappearance of peak ~ 450 nm on 30 s of exposure of W bulb light

Table 3 Degradation profile of Methyl Orange (MO) dye with Li₂Mn_1−xRu_xO₃ and MnO₂ in different experimental conditions

In Figure 12, we have plotted the absorption of the MB solution mixed with Li₂Mn_0.9Ru_0.1O₃ composition varied time (b) 1 min (c) 2 min (d) 3 min exposure in visible light. Methylene blue show absorption band at high energy due to π-π* transition of benzene ring while low energy band ~ 660–670 nm corresponds to n–π* transitions (where n is the free doublet on the nitrogen atom of C=N bond and free doublet of S atom on S=C bond). Insets show the zoomed plot in the wavelength range 400 to 800 nm for corresponding time exposure in visible light. After 1 min of exposure in visible light, MB peak (λ = 663 nm) shifts to 619 nm and absorption is very much decreased. After 3 min exposure of visible light, MB is degraded. As prepared MB solution shows no degradation with ultrasonication (Table 4). We have also recorded the degradation using MB solutions (pH6) exposed in Tungston bulb light for 60 min each with (b) Li₂MnO₃, where Mn is present in +4 valence state (c) LiMn₂O₄, where Mn is present in +3 and +4 valence states and (d) MnO₂, where Mn is present in +4 valence state. We have found that there is no substantial degradation of MB solution with Li₂MnO₃ and LiMn₂O₄ while MnO₂ degrades partially in 60 min.

Fig. 12

Absorption spectra of the MB solutions during the decomposition reaction catalyzed by Li₂Mn_0.9Ru_0.1O₃ in the exposure of visible light for (b) 1 (c) 2 and (d) 3 min

Table 4 Degradation profile of methylene blue (MB) dye with Li₂Mn_1−xRu_xO₃ and MnO₂ in different experimental conditions

In Figure 13, we show the efficiency for MB in presence of (i) Li₂Mn_0.9Ru_0.1O₃, where Mn is present in mixed valence state +3 and +4, and Ru is in mixed valence state +4/+ 5 (ii) Li₂MnO₃, where Mn is present in +4 valence state and (iii) LiMn₂O₄ where Mn is present in +3 and +4 valence states. Li₂Mn_0.9Ru_0.1O₃ (Mixed valences containing oxide) shows very fast decomposition of Methylene blue at nominal pH 6, which was not observed with this efficiency earlier in this type of crystalline materials [40].

Fig. 13

Degradation rate of MB solutions [5 ml of 10 mg/L concentration] under visible light irradiation with 5 mg Li₂Mn_0.9Ru_0.1O₃, Li₂MnO₃ and LiMn₂O₄ each

Synchrotron X-ray diffraction analysis and Raman spectroscopy measurements show the change in structure with the doping of Ru at Mn site in Li₂Mn_1−xRu_xO₃ [x = 0.05, 0.1]. XRD studies show the change in lattice which suggest the presence of Ru with Mn in the lattice of Li₂Mn_1−xRu_xO₃ [x = 0.05, 0.1] systems. XAS studies show the presence of Ru⁺⁴/Ru⁺⁵ and Mn⁺³/Mn⁺⁴ redox couples in Ru doped materials. When Ru⁺⁴ doped at Mn⁺⁴ site, due to comparable redox potential of Ru⁺⁴/Ru⁺⁵ (1.07 eV) with that of Mn⁺³/Mn⁺⁴ (1.02 eV), Ru⁺⁴ (0.62 Å) converts into Ru⁺⁵ (0.565 Å) and for charge valence same amount of Mn⁺⁴ (0.53 Å) converts into Mn⁺³ (0.645 Å) [11, 15]. When Ru⁺⁴ doped at Mn⁺³ site in LaMnO₃ then mixed valence is not observed as shown in Fig. 7 and it also show by patra et al. [21]. It has been shown earlier that Mn⁺³(0.645 Å) (in octahedral coordinated configuration) has comparatively bigger radius than Mn⁺⁴ (0.53 Å) and it has an electron in e_g orbital. Due to one e_g electron, Mn⁺³–O₆ octahedral elongate in z axis direction and it brings distortion in lattice. Although, the presence of Ru⁺⁵ (0.565 Å) make up charge balance but due to size mismatch with Mn⁺³ (0.645 Å) could not stop distortion in lattice of Li₂Mn_1−xRu_xO₃ [x = 0.05, 0.1]. Raman and IR spectroscopic studies show the evidence of lattice distortion by changing the peak position and splitting in Ru doped compositions.

Impedance has been observed 10² times less in Li₂Mn_0.9Ru_0.1O₃ in comparison to that of Li₂MnO₃ due to the bigger d orbitals of Ru in comparison to Mn and mixed valences Ru⁺⁴/Ru⁺⁵ and Mn⁺³/Mn⁺⁴. Electrons can easily hope in Ru assisted invironment in Li₂Mn_1−xRu_xO₃ in comparison to Li₂MnO₃. Mn⁺³ contains 3 electrons in t_2g orbitals while 1 electron in e_g orbitals while Mn⁺⁴ contains only 3 electrons in t_2g orbitals. Ru⁺⁴ contains 2 paired and 2 unpaired electrons in t_2g orbitals while Ru⁺⁵ contains only 3 electrons in t_2g orbitals. One e_g electron of Mn⁺³ can hope to vacant e_g orbitals Mn⁺⁴ and Ru⁺⁵ in Ru doped Li₂MnO₃ compositions. One e_g electron of Mn⁺³ and presence of 4d orbitals containing Ru modify the impedance and capacitance properties of Ru doped Li₂MnO₃ compositions.

Diffuse reflectance spectroscopic studies suggest the optical absorption in visible light in Li₂Mn_1−xRu_xO₃ [x = 0.0, 0.05, 0.1] systems. Band gap increases with the increase of Ru content while crystallite size decreased. In the crystalline environment of Ru and Mn in Li₂Mn_1−xRu_xO₃ [x = 0.05, 0.1], Mn intersite and Ru intersite transitions using e_g − e_g, t_2g − e_g may become allowed by spin orbit coupling, lattice disorder/distortions and mixing of odd parity wave functions. Decreases in crystallite size of the compositions, Mn–Mn intersite, Ru-Mn intersite and spin orbit coupling are the main cause of enhanced band gap of Li₂Mn_0.9Ru_0.1O₃ and Li₂Mn_0.95Ru_0.05O₃ in comparison to Li₂MnO₃.

Presence of mixed valences Ru⁺⁴/Ru⁺⁵ and Mn⁺³/Mn⁺⁴ assist the process of degradation of MO and MB solutions with faster rate in comparison with MnO₂. Compounds LaMn_1−xRu_xO₃ degrades MO dye only at pH 2.5 and takes 15 min [21]. LaMn_1−xRu_xO₃ shows the presence of only Mn⁺³ and Ru⁺⁴ valence states. Our results show the presence of redox couples Mn⁺³–Mn⁺⁴ and Ru⁺⁴–Ru⁺⁵ valence states in Li₂Mn_1−xRu_xO₃ [x = 0.05, 0.1,]. Multiple valence states enhance the process of degradation of 5 ml each of MO and MB dyes in Li₂Mn_1−xRu_xO₃ [x = 0.05, 0.1,]. 5 mg of Li₂Mn_0.9Ru_0.1O₃ degrades 5 ml of MO (10 mg/L) up to 100% in 5 min and 5 ml of MB (10 mg/L) in 3 min at pH 6. 5 mg of MnO₂ does not degrade 5 ml of MO (10 mg/L) in 60 min and 5 ml of MB (10 mg/L) in 60 min at pH 6[Tables 3,4; Fig. 13]. 20 mg of flower like Mn₃O₄ (where Mn in Mn⁺³ valence state) degrades 50 ml MB (10 mg/L) up to 80% in 180 min in presence of H₂O₂ [8]. 100 mg nano wires of MnO₂ (where Mn in Mn⁺⁴ valence state) degrades 200 ml MO (40 mg/L) up to 100% in 90 min [9].

Possible degradation mechanism can be summarised by following reactions as shown by Ranjeh et al.:

$${\text{Methylene}}\;{\text{Blue}}/{\text{Methyl}}\;{\text{Orange}}\;{\text{Dye}} + \left( {{\text{O}}_{2}^{ - } /^{.} {\text{OH}}/{\text{H}}_{2} {\text{O}}_{2} } \right) \to {\text{Degraded}}\;{\text{water}}\;{\text{pollutant}}$$

Presence of multiple valence states Mn⁺³/Mn⁺⁴ and Ru⁺⁴/Ru⁺⁵ create photo generated electrons and holes by transferring electrons from valence band to conduction band leaving holes in valence band. Electrons react with O₂ and generate O₂⁻. O₂⁻ further reacts with H⁺ ions and generates H₂O₂. Holes react with OH⁻ and generate OH^• free radicals. So (O₂⁻/.OH/H₂O₂) react with MO, MB dyes solution and decolourize. If H⁺ ions are not present then these compounds does not degrade MO and MB dyes. In presence of H⁺ and visible light, Li₂MnO₃ does not degrade MO and MB dyes up to 24 h. Li₂Mn_1−xRu_xO₃ [x = 0.05, 0.1] degrades MO and MB dyes very fast in presence of H⁺ (pH 6) and W bulb/sunlight due to presence of mixed valence states Mn⁺³/Mn⁺⁴ and Ru⁺⁴/Ru⁺⁵. These materials degrades MB dye very fast in compare to other Mn based systemsMn₃O₄, MnO₂ and nano Li₂MnO₃ [8, 9, 43]. Easy material preparation of Li₂Mn_1−xRu_xO₃ and reusability of these materials is an advantage over other known TiO₂ based catalysts.

4 Conclusions

We have optimized the synthesis conditions for inducing the redox interaction between Ru⁺⁴ and Mn⁺⁴ which further results Mn⁺³–Mn⁺⁴ and Ru⁺⁴–Ru⁺⁵ valence states in Li₂Mn_1−xRu_xO₃ [x = 0.05, 0.1]. These redox couples in presence of H⁺ ions and W bulb/sunlight enhances the dye degradation process in Ru doped Li₂MnO₃. Li₂Mn_0.9Ru_0.1O₃ composition (i) shows 10² times decreased impedance and (ii) quickly degrades methyl orange and methylene blue solutions at pH 6 which shows its capability as effective dye degradation material which otherwise was neither found in other Mn based systems nor in these types of monoclinic crystals. These Ru doped Li₂Mn_1−xRu_xO₃ materials can be use for fast decomposition of other organic dyes which are harmful for environment. Fast degradation of dyes, easy material preparation and reusability can make these materials first choice in industry over other known TiO₂ based catalysts.