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NANOSYSTEMS:
PHYSICS, CHEMISTRY, MATHEMATICS Original article
Chupakhina T.I., et al. Nanosystems: Phys. Chem. Math., 2024,15 (4), 540-547.
http://nanojournal.ifmo.ru DOI 10.17586/2220-8054-2024-15-4-540-547
Study of photocatalytic activity in two light ranges of Sr2Mno.4Tio.eO4 oxide with the K2NiF4-type structure
Tatyana I. Chupakhina1", Olga I. Gyrdasova1'6, Anastasya M. Uporova1, Larisa Y. Buldakova1, Mikhail Y. Yanchenko1, Dzhavid V. Mamedov2, Yulia A. Deeva1, Inna V. Baklanova1
1 Institute of Solid State Chemistry UB RAS, Ekaterinburg, Russia
2Zavoisky Physical-Technical Institute, FRC Kazan Scientific Center of RAS, Kazan, Russia
°[email protected], [email protected] Corresponding author: Tatyana I. Chupakhina, [email protected]
PACS 82.65.+r
Abstract The photocatalytic properties of single-phase sample Sr2Mno.4Tio.6O4 is studied as a representative of a series of Sr2MnxTi1-xO4 solid solutions (x = 0.05, 0.15, 0.25, 0.4) obtained by the SHS. The sample annealed at 1200 °C is characterized by a uniform distribution of Sr, Tiand Mn in the oxidation degree (4+) inside the aggregates, the average size of which does not exceed 1 ^m. According to UV-Vis-NIR spectroscopy data, a narrowing of the band gap of Sr2TiO4 from 3.16 to 1.8 eV is observed when it is doped with 40 mol% of manganese. This is due to the high photoactivity of Sr2Mn0 4Ti0 6O4 in the HQ oxidation reaction in UV and blue light.
Keywords nanostructured strontium titanate, Raddlesden-Popper structures, oxidative photocatalysis Acknowledgements The work was supported by RCF, project No. 24-23-20123. The UV-Vis-NIR spectrum was recorded on equipment of the Center for Joint Use "Spectroscopy and Analysis of Organic Compounds" at the Postovsky Institute of Organic Synthesis, UB RAS.
For citation Chupakhina T.I., Gyrdasova O.I., Uporova A.M., Buldakova L.Y., Yanchenko M.Y., Mamedov D.V., Deeva Y.A., Baklanova I.V. Study of photocatalytic activity in two light ranges of Sr2Mn0.4Ti0. 6O4 oxide with the K2NiF4-type structure. Nanosystems: Phys. Chem. Math., 2024,15 (4), 540-547.
1. Introduction
The complex oxide Sr2TiO4 with layered Raddlesden-Popper structure (An+iB„O3n+i, n = 1, structural type K2NiF4) have high dielectric constant and low dielectric loss angle tangent. Strontium titanates are wide-gap semiconductors which are used in various fields of electronic engineering: electro-optical devices, multilayer capacitors and thermistors [1]. Sr2TiO4-based compounds find practical application in photosystems [2,3] as photocatalysts in the process of H2 evolution from H2O [4] or oxidation of toxic organic compounds to carbon dioxide and water [5]. Phenol derivatives are life-threatening pollutants of water resources. Titanium dioxide as a photocatalyst in the form of anatase/rutile
structural mixture is traditionally used to decompose benzene-containing pollutants [6] and 1.4-dihydroxybenzene (hy-droquinone) (60 %) n-benzoquinone and catechol are the main intermediates during phenol photooxidation by TiO2 catalyst [7]. The hydroquinone oxidation reaction may be considered as a model process for photooxidation reaction of phenolic compounds. The main problem of using TiO2 in photostimulated processes for removal of toxic contaminants is its limited spectral range of operation (UV region).
Structural analogs of TiO2 in the anatase phase are strontium titanates. The band gaps of Sr2TiO4 and TiO2 are comparable 3.3 eV) [8]. Therefore, compounds based on Sr2TiO4 can become an alternative to the known catalysts (Degussa). At the same time, crystalline Sr2TiO4, like TiO2, shows photoactivity only under irradiating by light with wavelength Amax = 253 nm (UV range) [8].
There is extensive information on methods of surface activation of perovskite-like oxides with description of various synthesis technologies, creation of composite structures, and design of heterostructures [9-14]. The main way to expand the spectral working range of strontium titanates is doping with d-metals in position B, as well as increasing the working surface area [9-13]. It is known that doping with transition elements like Mn, leads to enhanced light absorption ability of strontium titanates due to the effective reduction of the band gap. Thus, doping of perovskite-like structure strontium titanate with 1 and 3 at.% Mn increases its photocatalytic activity in the decomposition of methylene blue, and the oxide with lower Mn content is more effective as a catalyst [13]. In contrast, [14] found that in SrTi1-xMnxO3 (x = 0.05 -0.2), the sample with x = 0.15 was found to be a more efficient catalyst for the decomposition of methylene blue.
© Chupakhina T.I., Gyrdasova O.I., Uporova A.M., Buldakova L.Y., Yanchenko M.Y., Mamedov D.V., Deeva Y.A., Baklanova I.V., 2024
There is no information on the application of manganese-containing layered perovskites based on Sr2TiO4 in photo-catalysis. There are also no studies in the field of using doped strontium titanates with the structure of the first and second homologues of the Radlesden-Popper series as photocatalysts for the decomposition of phenolic compounds.
Thus, the goal was to synthesize Sr2Ti1-xMnxO4 (0.05 < x < 0.4) solid solutions and investigate their activity in the photooxidation of 1.4-dihydroxybenzene (hydroquinone).
By changing the doping of Sr2TiO4 and the conditions of its production, it is possible to extend its photosensitivity to the visible range of spectrum.
Practically significant properties of such materials (specific surface area, degree of defectivity, porosity, size, morphology and internal architecture of aggregates) are set by synthesis. The authors propose a comprehensive solution to these problems by precursor synthesis method The technology of salt composites pyrolysis was used for preparation of complex oxides with various composition (method of self-propagating high-temperature synthesis, SHS) [15,16].
In this work, complex oxide Sr2Mn0.4Ti0.6O4 with K2NiF4-type structure was generated by SHS method. Its catalytic properties were studied in the photooxidation reaction with 1.4-dihydroxybenzene (hydroquinone, HQ) when the sample was exposed to radiation with wavelengths of Amax = 253 nm (UV range) and Amax = 440 - 460 nm (visible blue).
2. Experimental
Reagents used are as follows: strontium nitrate Sr(NO3)2 (high purity), titanium tetraisopropoxide Ti(i-OC(CH3)2)4 (p = 0.963 g/cm3) and manganese (II) nitrate Mn(NO3)2 • 4H2O (high purity). C2H5OH 96 % and HNO3 (high purity) were used as solvents. As an organic reducing agent for the initiation of the fuel process in a mixture of nitrates, (NH4)2C6H6O7 (high purity) was used. Sr2Mn0 4Ti0 6O4 was prepared by SHS method according to the procedure described in [16]. The final single-phase oxide Sr2MnxTi1-xO4 was obtained by annealing the SHS product at a temperature of 1200 °C during 8 h.
X-ray powder diffraction (XRD-7000 diffractometer, Shimadzu, Japan) using CuKa radiation in the 26 range 5 to 80° with a step of 0.03° was employed. The morphological peculiarities were examined by the scanning electron microscopy method (SEM) on a Tescan Vega Compact microscope with an EDS X-max Oxford Instruments. To confirm the change in band gap, the UV-Vis-NIR spectrum in the wavelength range of 190 - 1400 nm (BaSO4 was used as the standard) of compound Sr2Mn0 4Ti0 6O4 was recorded using Shimadzu UV-2600 (Japan) spectrophotometer.
HQ working solution was prepared immediately before the experiment. 1.0 cm3 of the initial hydroquinone solution with the concentration of C(C6H4(OH)2) = 1-10-2 mol/dm3, 1.5 cm3 of 0.5 M Na2SO4 solution were placed in 25.0 cm3 measuring flasks and brought to the mark with distilled water. The concentration of the working solution of hydroquinone is 4.0 - 10-4 mol/dm3. Before the work, a calibration graph was fixed in the range of HQ concentrations 0.0 - 4.0-10-4 mol/dm3 by measuring the height of the anodic oxidation peak of HQ at 0 V. The solutions were transferred to a cell in which 50 mg of photocatalyst was previously added and exposed to radiation. UV irradiation of HQ solutions was carried out in quartz cells using a BUV-15 lamp (Amax = 253 nm). Oxidation under the blue lamp (Amax = 440 -460 nm) was carried out in glass cells. The HQ content in the solution was controlled by voltammetry.
Voltammetric study of Sr2Mn0 4Ti0 6O4 samples was carried out with use of an electroactive carbon paste electrode (CPE). It consisted of a mixture of a substance to study, spectrally pure graphite with particles' size of < 63 ^m, and a binder (Vaseline oil), taken in a 1: 9 : 3 weight ratio and had the working area of 0.07 cm2. The reference electrode was a saturated silver-silver chloride electrode. The voltammograms were recorded in two polarization modes of CPE, in 0.5 M Na2SO4. Redox processes on the surface of the samples were investigated as follows. The reduction processes were fixed from +0.6 to -1.6 V, the oxidation processes were recorded from -1400 to +600 V. All measurements were realized by PU-1 polarograph at scan rate 30 mV/s.
3. Results and discussion
In the studied system of solid solutions Sr2MnxTi1-xO4 (T = 1200 °C; x = 0.05, 0.15, 0.25, 0.4) according to XRD data, only the composition Sr2Mn0 4Ti0 6O4 is single phase (Fig. 1). The other samples contain SrO and TiO2 as impurity phases.
In [14], during the preparation of Mn-dopping SrTiO3, it was noted the presence of similar impurity phases, which was attributed by the authors to the formation of strontium carbonate. However, these impurities disappear at the highest possible Mn concentration, indicating that doping promotes the complete reaction. The absence of the MnO peak in the diffractograms means that it takes place the successful introduction of Mn into the lattice of SrTiO3 [14]. In addition, the authors suggested the different valence state of manganese during the decomposition of the carboxylate complex, especially noticeable at small amounts of dopant [17,18].
For further study, we have chosen a single-phase sample of Sr2Mn0 4Ti0 6O4. The diffractogram of the sample Sr2Mn0 4Ti0 6O4, indexed in space group I4/mmm (No. 139) is presented in Fig. 1b. Crystallochemical characteristics of Sr2TiO4 and Sr2Mn0 4Ti0 6O4 are summarized in Table 1.
The decrease of the cell parameters with decreasing the unit cell volume of doped strontium titanate Sr2Mn0 4Ti0 6O4 confirms the incorporation of the smaller Mn4+ ion (0.53 A) into the octahedral positions occupied by the larger Ti4+ ion
Table 1. The crystallochemical parameters of Sr2TiO4 and Sr2Mno.4Tio.6O4 (space group I4/mmm, Sr (4e, 0; 0; z), Ti/Mn (2a, 0; 0; 0), O1 (4e, 0; 0.5; 0), O2 (4c, 0; 0; z))
Sr2TiO4 Sr2 Mn0.4Ti0.6O4
Cell parameters
a, A 3.883(4) 3.846(1)
c,A 12.588(2) 12.562(5)
V, A3 189.894(3) 185.882(1)
Atom parameters
Sr
z,A 0.354(7) 0.354(1)
O1
Occupancy 0.979(5) 1.02(4)
O2
z,A 0.159(1) 0.160(1)
Occupancy 1.078(5) 1.01(4)
r. A 2
Sr 0.317(2) 0.766(9)
Mn — 0.616(2)
Ti 0.334(5) 0.616(2)
O1 0.546(1) 1.074(4)
O2 0.548(2) 1.433(4)
Selected bond lengths (A)
Ti/Mn-O1 (x4) 1.941(2) 1.923(1)
Ti/Mn-O2(x2) 2.003(5) 2.018(8)
Sr - O1 (x4) 2.673(7) 2.653(7)
Sr - O2a (x4) 2.751(3) 2.726(5)
Sr - O2b (x4) 2.454(5) 2.434(3)
R-factors
Rwp, % 16.4 8.14
Rexp, % 11.6 10.13
Rp, % 10.6 5.79
Rf 3.83 3.82
X2 1.99 0.72
(a) HwiJU. . w«JasJi»< . A . Ji x=0.05 i,_LA,
*_______________kl. it L.i/u .. ilL. I ._ . M x=0.15 I 1A
M»»JLt.. ..........1____L _ J . JI . ii5
1 . 1 1 _________At 1 Jul______Xa°'a
MMlllMlllllllllllMlllMMl
10 20
30
40 50
20, degree
60
70
Fig. 1. (a) Diffractograms of Sr2MnxTi1-xO4 (x = 0.05, 0.15, 0.25, 0.4); (b) Experimental ('◦'), theoretical (black line) and difference (blue line) diffractograms of Sr2Mno.4Tio.6O4, ('|') are Miller indices
(0.605 A) [19]. According to SEM data, the pyrolysis product of the reaction mass Sr2Mn0.4Ti0.6O4 consists of nanoscale particles (Fig. 2a).
After annealing in the indicated temperature range, agglomeration of crystallites up to 1 nm is observed. The results of EDAX study confirm the chemical composition of the compound. The uniform distribution of elements in the agglomerates is confirmed by EDS-mapping (Fig. 2b).
j iBMfliasMH
Fig. 2. SEM image of SHS-synthesis product (a) and EDS-mapping of Sr2Mn0.4Ti0.6O4, calcined at 1200 0C (b)
To establish the effect of the manganese dopant on the optical characteristics of Sr2 TiO4, the optical absorption spectrum in the UV, visible and near-IR ranges was studied (Fig. 3a).
The absorption band at 459 nm is attributed to the 4A2g T2g transition of the Mn4+ ion, and the shoulder near 300 nm is most likely associated with the 4 A2g T1g transition of the Mn4+ ion (Fig. 3a). The determination of the band gap is performed by the way of approximation of the absorption edge using the Tauc relation: (ahv)2 = A(hv — Eg), where a is the absorption coefficient, hv is the photon energy, Eg is the band gap, A is an independent constant. According to this equation, the optical band gap can be obtained by the extrapolation of the linear part of the (ahv)2 = f (hv) to the intersection with the abscissa axis (Fig. 3b). The band gap for Sr2Mn0 4Ti0 6O4 is 1.8 eV.
To clarify the valence state of Mn in Sr2Mn0.4Ti0.6O4, EPR spectra and voltammetry (VA) data of the sample were investigated. To this date, there is no discussion about the nature and properties of Mn4+ centers in the STO crystal. It is generally accepted that tetravalent manganese ions isovalently replace the octahedrally coordinated position of titanium in the crystal. In this case, the ground state of Mn4+ ions is the orbital singlet 4A2, i.e., the outer electron shell is characterized by a highly symmetric distribution d of electron density [17,18]. In other words, Mn4+ ions should not
Fig. 3. (a) UV-Vis-NIR spectra of Sr2Mn0 4Ti0 6O4; (b) band gap Eg estimation using Tauc plot method
introduce a perturbation into the crystal lattice that would lead to a decrease in the symmetry of the center. Therefore, the structure of the EPR spectrum should reflect the symmetry of the crystal field.
Figure 4a shows the EPR spectrum of Sr2Mn0 4Ti0 6O4. The EPR spectrum is symmetric, well described by the Lorentz function. The parameters of the Lorentz curve are resonance field Hres « 3379.9 Oe, line width AH « 173.2 Oe, g-factor (having the value 1.99 which corresponds to the Mn4+ ion).
(O
5 so
-o
20 0 -20 -40 -60
(b>
sample ^—
. _ _ Mn4'=>Mn2* MnO? /-x
C
-12C0
e, mv
Fig. 4. (a) EPR spectra of Sr2Mn0 4Ti0 6O4; (b) Voltammetric recovery curves of Sr2Mn0.4Ti0 6O4, carbon and MnO2; background electrolyte -0.5 M NH4OH and 0.25 M Na2SO4
The value of g-factor is close to literature data for Mn4+ ions, both for highly concentrated and low concentrated compounds [20-23]. The absence of Mn4+ ion hyperfine structure in the EPR spectrum of Sr2Mn0 4Ti0 6O4 may be due to exchange interactions between manganese ions.
Voltammetry data confirm the presence of Mn4+ (Fig. 4b). The VA behavior of Mn(IV) oxide was preliminarily studied. At cathodic wave of MnO2 at -850 mV, one can see the reduction peak, that corresponds to the Mn(IV) reduction to Mn(II). A similar signal is recorded on voltammetry of Sr2Mn0.4Ti0.6O4 (Fig. 4b). From the analysis of the obtained polarization curves, it can be concluded that manganese in the studied sample Sr2Mn0 4Ti0 6O4 is initially in the form of Mn4+.
According to UV-Vis-NIR spectroscopy data, the narrowing of the band gap of Sr2Mn0.4Ti0.6O4 (1.8 eV) compared to Sr2TiO4 (3.16 eV) indicates a significant shift of the photosensitivity of the compound to the visible range of the spectrum.
The photocatalytic activity of the samples was evaluated by the decomposition rate of the reference organic compound 1.4-dihydroxybenzene (hydroquinone, HQ). The sample was found to exhibit high photocatalytic activity in both light ranges (Fig. 5).
The stability of the catalyst performance was tested in three consecutive oxidation cycles for 6 hours (Fig. 5b). The photoefficiency of Sr2Mn0 4Ti0 6O4 increases with increasing the number of cycles performed. Table 2 shows the values
mi
Fig. 5. Decrease in HQ concentration without catalyst (blank) and on Sr2Mn0.4Ti0.6O4 as a function of irradiation type and cycling for 6 h
of the rate constants of the oxidation reaction of HQ under UV and visible light. Kinetic equations for the 1st order reaction were used. The main kinetic parameters of the investigated sample were compared with the commercial catalyst Degussa P25. The obtained data show that the photoefficiency of the catalyst Sr2Mn0.4Ti0.6O4 exceeds Degussa P25 on average 4 times at UV stimulation (Table 2).
The increase in the rate of photooxidation and stabilization of photocatalytic properties of Sr2Mn0.4Ti0.6O4 with increasing time of catalyst operation in HQ working solutions can be related to activation of its working surface during the first oxidation cycle. Therefore, to increase the efficiency of the catalyst before use, it should be pre-soaked for 4 -6 hours in a working solution of hydroquinone. The approximate mechanism of photooxidation of organic substances on strontium titanates was discussed in a number of works [14-19]. Upon light stimulation of ZnO particles, photoinduced electrons migrate to the conduction band (CB), while photoinduced holes remain in the valence band (VB). The doping impurity cations (Mn) actively participate in the separation of the formed exciton e-/h+ pair. In addition, manganese directly interacts with oxygen adsorbed on the surface of Sr2Ti(Mn)O4. Other probable electron/hole trapping sites are charged oxygen vacancies (VO +, VO2+) [24]. As a result, various reactive oxygen species (ROS) such as hydroxyl and superoxide radicals (•HO and ^2—), extremely active oxidizing agents, are formed in the reaction zone. These ROS subsequently participate in the oxidation of HQ.
High photoactivity of Sr2Mn0.4Ti0.6O4 in HQ solutions indirectly indicates the formation of superoxide radical ^2 — on its working surface in the first place. In practice, the radical trap addition method is used to determine the type of
Table 2. Value of rate constants of HQ photooxidation reaction (ks) and HQ half-life (t) in the presence of photocatalysts Degussa P25 and Sr2Mn0.4Ti0.6O4
Catalyst Amax = 253 nm Amax = 440 - 460 nm
ks • 105, s-1 (n = 7, P = 0.95)* S • 105 ** £, % *** t , h ks • 105,s-1 S • 105 t , h
HQ blank. 1.34±0.12 0.11 8.9 14.4 0.15 0.03 132
Degussa P25 4.37±0.65 0.57 15.0 4.9 — — —
Sr2Mn0.4Ti0.6O4 Sr2Mn0.4Ti0.6O4
1st cycle 10.70±7,22 5.20 6.7 1.8 7.18±2.34 1.32 2.7
2nd cycle 18.62±2.86 1.15 15.3 1.0 8.09±4.37 3.52 2.4
3rd cycle 18.90±2.90 5.21 6.8 1.0 7.72±2.67 2.16 2.5
*Sample number n at confidence coefficient P;
**Standard deviation;
***Relative error of the average value of ks.
radicals. One of the most effective superoxide radical scavengers is the HQ derivative benzoquinone (BQ) [25]. According to our studies, the photoactivity of Sr2Ti1-xMxO4 (M = Mn, Cu) in the extended spectral range with respect to HQ oxidation exceeds essentially the known analogs [4-12]. Thus solid solutions Ti1-xMnxO2-x (0.003 < x < 0.016) with spherical aggregate morphology degrade HQ by 100 % in 11 h of operation (t = 5.5) in the UV and in 12 h (t = 6) with 90 % yield in blue light [26]. The use of Sr2TiO4-based solid substitution solutions reduces the oxidation time of aromatic compounds: for comparison, Sr2Mn0 4Ti0 6O4 decomposes HQ by 100 % in blue light in 5 h (t = 2.5). This result makes Sr2Ti1-xMnxO4 promising targets for photooxidative catalysis in the visible light range.
4. Conclusion
Mn-doped Sr2TiO4 samples were prepared by SHS method with different concentrations of Mn: 5, 15, 25 and 40 mol%. XRD results indicate that the sample with maximum manganese concentration of Sr2Mn0 4 Ti0.6O4 is the single phase in this system. According to the results of voltammetry, EPR, UV-Vis-NIR spectroscopy, Mn in the compound appears in the oxidation degree (4+), and the forbidden band of Sr2Mn0.4Ti0.6O4 narrows to 1.8 eV.
A study of the catalytic properties of Sr2Mn0 4Ti0 6O4 in HQ solutions showed its photoefficiency in the UV and visible light ranges during 3 consecutive photooxidation cycles. Moreover, the photooxidation rate of HQ upon irradiation with blue light is as close as possible to that in the UV range. These results are unexpected for Sr2TiO4 and require further investigation.
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Submitted 17 June 2024; revised 13 August 2024; accepted 14 August 2024
Information about the authors:
Tatyana I. Chupakhina - Institute of Solid State Chemistry UB RAS, Pervomayskaya, 91, Ekaterinburg, 620990, Russia; ORCID 0000-0002-1317-3803; [email protected]
Olga I. Gyrdasova - Institute of Solid State Chemistry UB RAS, Pervomayskaya, 91, Ekaterinburg, 620990, Russia; ORCID 0000-0002-0680-6094; [email protected]
Anastasya M. Uporova - Institute of Solid State Chemistry UB RAS, Pervomayskaya, 91, Ekaterinburg, 620990, Russia; ORCID 0009-0006-1219-8909; [email protected]
Larisa Y. Buldakova - Institute of Solid State Chemistry UB RAS, Pervomayskaya, 91, Ekaterinburg, 620990, Russia; ORCID 0000-0003-3642-7121; [email protected]
Mikhail Y. Yanchenko - Institute of Solid State Chemistry UB RAS, Pervomayskaya, 91, Ekaterinburg, 620990, Russia; ORCID 0000-0001-8306-906X; [email protected]
Dzhavid V. Mamedov - Zavoisky Physical-Technical Institute, FRC Kazan Scientific Center of RAS, Kazan, 420029, Russia; [email protected]
Yulia A. Deeva - Institute of Solid State Chemistry UB RAS, Pervomayskaya, 91, Ekaterinburg, 620990, Russia; ORCID 0000-0003-1207-7331; [email protected]
Inna V. Baklanova - Institute of Solid State Chemistry UB RAS, Pervomayskaya, 91, Ekaterinburg, 620990, Russia; ORCID 0000-0002-5643-7004; [email protected]
Conflict of interest: the authors declare no conflict of interest.
