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Correlating the Influence of Two Magnetic Ions at the A‑Site with the Electronic, Magnetic, and Catalytic Properties in Gd1−xDyxCrO3 Vikash Kumar Tripathi and Rajamani Nagarajan* Materials Chemistry Group, Department of Chemistry, University of Delhi, Delhi 110007 India S Supporting Information *

ABSTRACT: Considering the absence of reports dealing with the perovskite-structured orthochromites containing two A-site magnetic rare-earth ions, GdCrO3 and progressively Dy3+-substituted samples of the series Gd1−xDyxCrO3 have been synthesized employing the epoxide-mediated sol−gel procedure. The samples were characterized extensively using high-resolution powder X-ray diffraction, thermal analysis, Fourier transform infrared, Raman, and UV−visible spectroscopies, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) measurements. Monophasic samples possessing an orthorhombic perovskite structure emerged by calcining the xerogels formed by the reaction of rare-earth nitrates, chromium(III)chloride, and propylene oxide at 800 °C for 2 h. Uniform presence of wormlike morphology was observed in both the field emission SEM (FE-SEM) and TEM images of the samples. Zero-field and field-cooled magnetic measurements using a SQUID magnetometer down to 4 K showed that the Neel temperature of Gd0.5Dy0.5CrO3 was 155 K, more or less midway between the values observed for GdCrO3(169 K) and DyCrO3 (146 K). For the Gd0.5Dy0.5CrO3 sample, a spin reorientation was observed at ∼38 K when measured under an applied field. Because the optical band gap, determined by Kubelka−Munk function, of these chromites was around 3 eV, their application as a catalyst for the photodegradation of the aqueous rhodamine-6G dye solution was demonstrated, in which the percentage of the total dye that was degraded varied with the average ionic radius of A-site ions. A similar systematic trend was observed even for the catalytic oxidation of the XO dye in the presence of H2O2, with DyCrO3 influencing the reaction to a greater extent followed by Gd0.5Dy0.5CrO3 and GdCrO3. Both the photocatalytic and catalytic reactions followed pseudo-first-order kinetics. with increase in the ionic radius of rare-earth (RE3+), illustrating the influence of the A-site ion on the ordering of B-site ions. A lot of deliberations have been found to dominate the current literature, seeking the exact role of magnetic RE3+ for the observed canted antiferromagnetism and polarization in these systems.14−18 Studies dealing with sharing of the A-site with one magnetic ion (such as Gd or Pr) and the other nonmagnetic ion (La or Y) are quite common, whereas similar investigations with the A-site shared by magnetic rare earths are limited.19−29 Also, in such studies, the effects of only a smaller concentration of the second magnetic rare-earth ion (typically up to 10%) have been focused. These studies employed the solid-state diffusion process for preparing the samples.29 Given the superiority of solution-based methods over the solid-state reactions to efficiently cross the diffusion barrier rapidly at lower temperatures,30−34 here we present the epoxide-mediated sol−gel method for the generation of a continuous series of Gd1−xDyxCrO3 samples. Epoxide-mediated sol−gel process has been demonstrated to be an alternate and

1. INTRODUCTION Perovskite structure with the general formula ABX3 is very popular among solid-state chemists, condensed matter physicists, and materials scientists and has been investigated extensively.1−3 The innumerable number of possibilities of substitutions at A and B sites with ions of varying valence and ionic radii together with the choice of anion from halide to oxide adore it to exhibit many interesting properties such as high-temperature superconductivity, piezoelectricity, ferroelectricity, multiferroism, colossal magneto resistance, and catalytic functions.4−7 The recent demonstration of high solar cell capability in organic−inorganic halo perovskites has added a new dimension to the expanding list of multifunctions that one can anticipate to be discovered.8,9 The multifunctional character of rare-earth chromites (RECrO3) with a distorted perovskite structure has invited extensive investigation by the research community with special emphasis on the rare earths, influencing a possible magneto-electric coupling.5,10−13 Essentially, three different magnetic spin interactions, Cr3+−Cr3+, Cr3+−RE3+, and RE3+−RE3+, with isotropic, symmetric, and antisymmetric anisotropic exchange, respectively, exist in these systems.14−18 The antiferromagnetic Neel temperature (TN) arising from Cr3+−Cr3+ interactions has been found to increase © 2017 American Chemical Society

Received: April 19, 2017 Accepted: June 2, 2017 Published: June 14, 2017 2657

DOI: 10.1021/acsomega.7b00480 ACS Omega 2017, 2, 2657−2664

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efficient synthetic method compared to other sol gel route such as citrate sol gel or involving alkoxide.35 Further, it can be executed easily with high precision, beginning with air-stable inorganic salts without the use of any intricate setups; it will be a perfect method to study the solid solution of the type Gd1−xDyxCrO3, wherein the metal ions from different blocks of the periodic table with differing reactivities are present. Although alkoxides of rare earths are commercially available, it is difficult to control their rate of hydrolysis along with the hydrolyzable alkoxide of chromium, which may lead to variation in the final stoichiometry. Although a few studies state that gels could be formed only from GdCl3 and not from its nitrate salt, PrO2 has been successfully synthesized employing praseodymium nitrate, demonstrating the complex chemistry involved in the gelation of chemically equivalent metal ions.36,37 These differences illustrate the requirement of more studies employing the epoxide-based gel method so as to bring out more clarity. In the present study, GdCrO3, DyCrO3, and a complete series of the solid solution between them have been synthesized following the epoxide-mediated sol−gel method and have been characterized extensively using an array of analytical techniques including high-resolution powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), Raman spectroscopy, ultraviolet−visible (UV−vis) spectroscopy, and magnetic measurements. Interesting magnetic interactions and systematic variation of the catalytic efficiency emerged in the series Gd1−xDyxCrO3 demonstrated the influence of a second magnetic ion at the A-site to tailor its properties.

Figure 1. (a,b) TG and differential calorimetric traces of xerogels obtained from the reaction of nitrate salts of gadolinium (i), dysprosium (ii), and Gd/Dy (0.5:0.5) (iii) with chromium chloride with polypropylene oxide (PPO).

2. RESULTS AND DISCUSSION 2.1. Structure, Morphology, and Magnetic Properties. The intense green-colored xerogel obtained from the reaction of propylene oxide (PO) with the reactants was subjected to thermal analysis experiments. The results from this experiment are presented in Figure 1. The gradual weight loss with progressing temperature indicated the homogeneity of the gels. Whereas the mass loss till 120 °C was due to the removal of moisture and occluded water in the gels, the greater mass loss after 120 °C and up to 400 °C could be attributed to the decomposition of organic moiety from the gels. Both these steps were exothermic in nature. Epoxide-mediated gels show exothermic events for the observed mass losses in the differential thermal analysis (DTA) because epoxide linkages are quite weak and prone to moderately violent decomposition triggered by heat.39 Minimal weight loss beyond 750 °C with no observable thermal events suggested that the samples can be calcined at temperatures above this to obtain the mixed metal oxide. Pale green color powders were obtained on heating these samples at 800 °C for 2 h in a muffle furnace, followed by natural cooling to room temperature by switching off the furnace. PXRD patterns of these samples indicated the formation of the crystalline perovskite-structured oxide (Figure 2). Formation of any other secondary phase such as Cr2O3 was not observed, highlighting the homogeneous mixing and concurrent reaction of the constituents. The observed reflections were indexed in an orthorhombic symmetry, and successful Le Bail refinement of the PXRD pattern was carried out in the Pnma space group using TOPAS3 software.38 The refined patterns and the extracted lattice constants are provided in Figure S1 and Table 1, respectively. The subtle variation in the ionic sizes of Gd3+ and Dy3+ (VI coordination 0.930 and 0.912 Å, respectively) because of the lanthanide contraction

Figure 2. PXRD patterns of the calcined products from xerogels obtained from the reaction of gadolinium nitrate, dysprosium nitrate, and chromium chloride with their relevant ratio of composition and PPO at 800 °C for 2 h. (a) GdCrO3, (b) Gd0.9Dy0.1CrO3, (c) Gd0.7Dy0.3CrO3, (d) Gd0.5Dy0.5CrO3, (e) Gd0.3Dy0.7CrO3, (f) Gd0.1Dy0.9CrO3, and (g) DyCrO3.

was evident from the shift of all diffraction peaks toward the higher 2θ values for the Dy-substituted samples as compared with GdCrO3 (Figure 2). All three lattice constants of the orthorhombic unit cell showed a linear variation with the increase in the concentration of Dy replacing Gd in the lattice, as shown in Figure 3. This was in accordance with the Vegard’s law of the solid solution of two constituents. The uniform presence of wormlike morphology was observed in the field emission scanning electron microscopy (FE-SEM) images of GdCrO3, DyCrO3, and Gd0.5Dy0.5CrO3 (Figure 4a−c). Apparently, this kind of morphology is quite typical of samples synthesized either employing polyhydroxy alcohol or through 2658


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the formation of a metal hydroxide precursor, as reported earlier in case of GdFeO3 and La0.5Sm0.5CrO3 samples.31,32,40 The wormlike morphology was also observed for Gd0.5Dy0.5CrO3 sample in its TEM image (Figure 4d). From the energy-dispersive X-ray (EDX) analysis of this sample, Gd/ Dy/Cr was found to be in a ratio of 0.5:0.5:1, close to the starting nominal composition (Figure 4e). This illustrated the effectiveness of this synthetic method in minimizing the loss of any metal ions during processing and thus aiding in the preservation of the nominal composition in the final product. The Fourier transform infrared (FTIR) and Raman spectra of GdCrO3, DyCrO3, and Gd0.5Dy0.5CrO3 are presented in Figure 5a,b, respectively. In the FTIR spectra, band present at around

Table 1. Summary of Extracted Lattice Parameters of GdCrO3, Gd0.9Dy0.1CrO3, Gd0.7Dy0.3CrO3, Gd0.5Dy0.5CrO3, Gd0.3Dy0.7CrO3, Gd0.1Dy0.9CrO3, and DyCrO3 after Successful Le Bail Refinement of Their PXRD Patterns lattice parameter (Å) composition GdCrO3 Gd0.9Dy0.1CrO3 Gd0.7Dy0.3CrO3 Gd0.5Dy0.5CrO3 Gd0.3Dy0.7CrO3 Gd0.1Dy0.9CrO3 DyCrO3

a 5.31460 5.31202 5.31011 5.29802 5.29070 5.29001 5.28930

b (39) (80) (87) (94) (73) (67) (93)

5.52831 5.52354 5.52340 5.52090 5.51980 5.51920 5.51600

c (44) (65) (74) (31) (67) (39) (64)

7.60735 7.60100 7.59501 7.59210 7.58911 7.58402 7.58110

(68) (72) (62) (85) (60) (95) (73)

Figure 3. Plot of orthorhombic cell parameters, a, b, and c, vs Dy content in Gd1−xDyxCrO3.

Figure 5. (a,b) Relevant portions of FTIR and Raman spectra of GdCrO3, GdCrO3, and Gd0.5Dy0.5CrO3 samples.

Figure 4. (a,b) EDX spectrum and TEM image of Gd0.5Dy0.5CrO3. (c−e) FE-SEM images of Gd0.5Dy0.5CrO3, DyCrO3, and GdCrO3, respectively. 2659


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430 cm−1 were due to the O−Cr−O deformation vibrations. The peaks at around 480 and 660 cm−1 corresponded to the Ln−O and Cr−O stretching vibrations (Figure 5a).41 As compared with the FTIR spectra, bands in the Raman spectra are quite sensitive to the introduction of the second rare-earth ion at the A-site.42,43 Following the group theoretical predictions for an orthorhombic structure with Pnma space group with four formula units per unit cell, 24 vibration modes are Raman-active, namely, 7Ag + 5B1g + 7B2g + 5B3g are possible. Four different types of distortions in the structure, namely, rotation of CrO6 octahedra, Jahn−Teller distortion, and shift of Ln atom from its position, account for the appearance of Raman modes. In the Raman spectra of GdCrO3, DyCrO3, and Gd0.5Dy0.5CrO3, a maximum of eight modes were observed other than the band at around 830 cm−1 (Figure 5b). We did not observe other bands because they were very weak to be observed or at very low wave numbers below the current experimental cutoff. The sharp and intense modes below 200 cm−1 arise primarily from Ln-ion vibrations. With progressive substitution of Dy for Gd, all of the observed modes, except a few, shifted their positions to higher values as compared with the observed values for GdCrO3. The two important modes A1g and B2g, arising from the distortion caused by the size of the rare-earth ion on the CrO6 octahedra, at 550 and 325 cm−1 became sharp on the introduction of Dy for Gd in the structure, confirming its inclusion at the crystallographic site. This was in accordance with earlier reports, wherein the data in the range have been provided up to 600 cm−1.12 The bands observed beyond this range have been reasoned out to indicate the higher oxidation state of chromium in the samples.42,43 This was correlated with the oxidizing nature of epoxide as gelation agent for the synthesis of these samples. Epoxide linkages are known to cleave exothermically when heated, and the exothermic events observed in the differential scanning calorimetry (DSC) experiments of xerogels of these samples suggest that smaller amounts of chromium in the structure may be oxidized to higher states in addition to +3 during bulk synthesis. In general, three types of magnetic interactions dominate in rare-earth orthochromites.14−18 Whereas the Cr−Cr interaction dominates in determining the magnetic properties above 100 K, the Gd−Cr interaction defines the magnetic properties below 100 K. The Gd−Gd interaction results in ordering at around 2.3 K. At the Neel temperature, a paramagnetic to antiferromagnetic phase transition can be observed, which is attributed to the Cr3+−Cr3+ exchange. This is greatly affected by the rare-earth ionic radius that can couple with the Cr3+ spin structure. The orientation of the rare-earth moments with respect to Cr3+ spin depends upon the nature of the effective magnetic field at the rare-earth site. Anisotropic magnetic interactions between Ln3+ and Cr3+ ions are known to result in spin reorientation. Such behavior is favorable for inducing ferroelectric polarization. The temperature dependence of the molar susceptibility of GdCrO3, DyCrO3, and Gd0.5Dy0.5CrO3 from the SQUID measurements carried out under zero fieldcooled (ZFC) and field-cooled (FC) conditions are presented in Figure 6. From the ZFC measurements, TN values for GdCrO3, DyCrO3, and Gd0.5Dy0.5CrO3 were extracted. Although the TN values for GdCrO3 (169 K) and DyCrO3 (146 K) were close to the values reported earlier,14−18 it was 155 K for the Gd0.5Dy0.5CrO3 sample, suggesting its high homogeneity in terms of the presence of two magnetic ions at the A-site. For GdCrO3, it can be observed that magnetization

Figure 6. Temperature dependence of the molar susceptibility curve (χ−T) of (a) GdCrO3, (b) GdDyCrO3, and (c) Gd0.5Dy0.5CrO3 samples measured at an applied magnetic field of 100 Oe under ZFC, FC (inset), and TN (inset) between 3 and 300 K.

becomes negative with the decrease in the temperature. Such intriguing magnetic reversal behavior has been reported earlier.31 GdCrO3 and DyCrO3 showed paramagnetic behavior at low temperatures beyond TN. For the Gd0.5Dy0.5CrO3 sample, a spin reorientation is observed at ∼38 K in the FC measurements. Such a trend has been reported earlier for SmCrO3 at 33 K.31 The observance of such spin reorientation highlighted the emergence of interesting magnetic interaction induced by incorporating two different magnetic ions at the Asite. 2.2. Optical, Catalytic, and Photocatalytic Properties. The diffuse reflectance spectra of GdCrO3, Gd0.9Dy0.1CrO3, Gd 0 . 7 Dy 0 . 3 CrO 3 , Gd 0 . 5 Dy 0 . 5 CrO 3 , Gd 0 . 3 Dy 0 . 7 CrO 3 , Gd0.1Dy0.9CrO3, and DyCrO3 and their absorbance data in terms of Kubelka−Munk (K−M) function are presented in Figures 7 and 8, respectively. In the UV−vis diffuse reflectance data of GdCrO3 and DyCrO3, a sharp band at 274 nm was present, signifying the 8S7/2 → 6I11/2 transition of the Gd3+ ion. Other maxima at 390, 800, and 901 nm were attributed to the transitions from the ground state 6H15/2 to 4I13/2, 6F5/2, and 6 F7/2 of the Dy3+ ion.34,44−47 Additionally, the DMSO-d6 transition of Cr3+ was observed at 456 and 615 nm. The parity-forbidden and spin-allowed transitions [4A2g (4F) to 4 T2g, 4T1g (4F), and 2A2g → 2T1g, and 2A2g → 2E2g] of the Cr3+ 2660


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Gd0.3Dy0.7CrO3, Gd0.1Dy0.9CrO3, and DyCrO3 (estimated from the plot of K−M function as the absorption coefficient and photon energy) was found to be 3.36 eV (for GdCrO3), 3.11 eV (for Gd0.9Dy0.1CrO3), 3.06 eV (for Gd0.5Dy0.5CrO3), 3.03 eV (for Gd0.3Dy0.7CrO3), 2.99 eV (for Gd0.1Dy0.9CrO3), and 2.96 eV (for DyCrO3). Although band gap value for DyCrO3 was slightly above the reported value of 2.8 eV (for nanoplates),47 the decreasing trend of the band gap with the substitution of rare earth of smaller size matched well with the literature.46 As the observed band gap values fell in the range of wide band gap semiconductors, it prompted us to investigate their role as catalysts for the degradation of aqueous rhodamine (Rh)-6G dye solution in presence of visible radiation. Additionally, their catalytic role for the oxidative degradation of the XO dye solution with H2O2 was examined.48,49 The results from the photocatalytic degradation of Rh-6G dye under visible light with varying durations of irradiation are presented in Figures 9 and S2. From the plots of C/C0 versus the

Figure 7. (a,b) UV−vis diffuse reflectance spectra and band gap estimation for GdCrO3 and DyCrO3 using the K−M function analysis.

Figure 9. Comparative temporal changes in the absorbance spectra of (Rh-6G). Black, olive, cyan, blue, red, and magenta circles show the photolysis of (a) GdCrO3, (b) Gd0.7Dy0.3CrO3, (c) Gd0.5Dy0.5CrO3, (d) Gd0.3Dy0.7CrO3, and (e) DyCrO3 samples, respectively.

irradiation time, it was observed that the percentage of Rh-6G dye degraded by about 70, 75, 78, 84, and 90% in the presence of GdCrO3, Gd0.7Dy0.3CrO3, Gd0.5Dy0.5CrO3, Gd0.3Dy0.7CrO3, and DyCrO3 samples, respectively, within 60 min. Assuming pseudo-first-order, the rate constants for these reactions were estimated to be 2.6 × 10−2, 3.52 × 10−2, 4.28 × 10−2, 6.24 × 10−2, and 8.12 × 10−2 min−1. This is exactly reverse to the trend observed in case of rare-earth orthoferrites.50 One of the possible reasons could be the existence of a narrow window of band gap in orthoferrites consisting of rare-earth ions of differing ionic radii compared with the reasonably wide window of band gap in rare-earth orthochromites. Lattice distortion has also been found to play a significant role in enhancing the photocatalytic property of oxide systems because it could provide additional route to trapping of holes and eventually decrease the recombination rate of electron−hole pairs. Although in perovskite oxides the valence and conduction bands are made up of 3d orbitals of B-ion and 2p orbitals of oxygen, the sensitivity shown in the present case of orthochromites suggested the active participation of the valence electronic structures of A-cations. Whereas Gd3+ has half occupation of f-orbital situation, its dilution with more than half-filled Dy3+ alters its band gap and catalytic activity. The oxidative degradation of the aqueous solution of the XO dye employing H2O2 as the oxidant and the synthesized samples as

Figure 8. (a,b) UV−vis diffuse reflectance spectra and band gap estimation for the Gd0.9Dy0.1CrO3, Gd0.7Dy0.3CrO3, Gd0.5Dy0.5CrO3, Gd0.3Dy0.7CrO3, and Gd0.1Dy0.9CrO3 samples using the K−M function.

ion were observed at 691 and 730 nm, respectively.34,44−47 All of these bands were present in the progressively Dy-substituted samples as well. The optical band gap of GdCrO 3 , Gd 0 . 9 Dy 0 . 1 CrO 3 , Gd 0 . 7 Dy 0 . 3 CrO 3 , Gd 0 . 5 Dy 0 . 5 CrO 3 , 2661


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efficiency corresponding to the change in the ionic radius was observed. A similar observation was made even when the samples were used as catalysts for the oxidative degradation of XO. These results validated the fact that not only the magnetic properties but also the catalytic and photocatalytic functions of these systems can be fine-tuned just by altering the A-site magnetic rare-earth ion. There appears to be an intrinsic cooperative effect between Cr3+ and Ln3+, affecting the catalytic properties of these systems. In our view, this can add another dimension to the host of multifunctions exhibited by orthochromites.

catalysts revealed that the degradation of XO was indeed very sluggish in the absence of a catalyst even after 45 min of reaction, as indicated by the slight decrease in the intensity of λmax at 434 nm (Figures 10 and S3). Increasing the amount of

4. EXPERIMENTAL SECTION 4.1. Synthesis. Gd2O3 (Alfa Aesar 99%), Dy2O3 (SigmaAldrich 99.9%), CrCl3·6H2O (99.5% CDH), absolute alcohol (Merck), and PO (Alfa Aesar) were used as purchased for the experiments. Gd2O3 (0.362 g, 1 mmol) and Dy2O3 (0.372 g, 1 mmol) were separately digested in a minimum amount of nitric acid to prepare the respective metal nitrate salts in situ. For the synthesis of xerogel, 0.266 g (1 mmol) of CrCl3·6H2O dissolved in absolute methanol was mixed with both the nitrate solutions, followed by the addition of 1.4 mL (20 mmol) of PO slowly under stirring. The mixture was sonicated for nearly 2 min, after which a green-colored thick gel formed immediately. For the synthesis of Gd1−xDyxCrO3, the following amounts of rare-earth oxides together with 0.266 g (1 mmol) of CrCl3· 6H2O were used: Gd2O3 (0.3256 g, 0.9 mmol) and Dy2O3 (0.0372 g, 0.1 mmol), Gd2O3 (0.2534 g, 0.7 mmol) and Dy2O3 (0.116 g, 0.3 mmol), Gd2O3 (0.181 g, 0.5 mmol) and Dy2O3 (0.186 g, 0.5 mmol), Gd2O3 (0.108 g, 0.3 mmol) and Dy2O3 (0.2604 g, 0.7 mmol), and Gd2O3 (0.0362 g, 0.1 mmol) and Dy2O3 (0.3348 g, 0.9 mmol). The gels were obtained in a way exactly similar to the one described for GdCrO3. 4.2. Characterization. PXRD patterns were recorded using a PANanalytical X’Pert diffractometer equipped with a PIXcel3D detector employing Cu Kα radiation (λ = 1.5418 Å) with a scan rate of 58.39 s/step and a step size of 0.01313° over the range of 2θ = 10−70° at 25 °C. The PXRD patterns were fitted using the Le Bail method to obtain the cell dimensions using TOPAS3 software.38 The FTIR spectra were recorded using a PerkinElmer 2000 FTIR spectrometer using KBr disks. The Raman spectrum was recorded using a Renishaw spectrometer via a microscope system operating with an Ar+ laser (λ = 488 nm). UV−vis diffuse reflectance spectra of the samples in a solid form were recorded using a UV−vis spectrophotometer (PerkinElmer Lambda 35) equipped with an integrating sphere. BaSO4 was employed as the reference. Thermogravimetric (TG) analysis of xerogels was carried out on a NETZSCH STA-449 F3 instrument in the temperature range of 30−800 °C at a heating rate of 10 °C/min. FE-SEM micrograph and EDX analysis of the sample were performed using Hitachi S3700M and JEOL 660LV microscopes, respectively. TEM analysis was carried out using an FEI Technai G2 20 electron microscope operating at 200 kV. Magnetic measurements of the powder samples were carried out using a superconducting quantum interference device magnetometer (MPMS XL Ever Cool) between 4 and 300 K. 4.3. Catalytic Experimental Details. Photocatalytic degradation was carried out in an immersion type, in-house fabricated reactor under visible radiation employing a mercury vapor lamp with 125 W power (Philips, India). Thirty milligrams of the catalyst was added to 50 mL of the aqueous dye solution of the dye Rh-6G with an initial concentration of

Figure 10. Comparative temporal changes in the absorbance spectra of XO. Black, olive, cyan, blue, red, and magenta circles show the without catalyst, (a) GdCrO3, (b) Gd0.7Dy0.3CrO3, (c) Gd0.5Dy0.5CrO3, (d) Gd0.3Dy0.7CrO3, and (e) DyCrO3 samples, respectively.

H2O2 did not alter the kinetics. However, a drastic reduction in the intensity of absorption peak was observed within 5 min after the addition of 30 mg of GdCrO3, Gd0.7Dy0.3CrO3, Gd0.5Dy0.5CrO3, Gd0.3Dy0.7CrO3, and DyCrO3 samples. The solution became completely colorless within 20 min, suggesting the catalytic role of the sample. To quantify this oxidation reaction, C/C0 was calculated after generating a linear calibration curve (a straight line) using the absorbance values of standard solutions of XO with different concentrations. The concentration of XO in the presence of a catalyst was then determined by fixing the absorbance versus the concentration graph. From the C/C0 plot with the reaction time, it can be seen that 76.8, 87.3, 90.0, 92.4, and 94.0% of XO degraded within 20 min of reaction in the presence of GdCrO3, Gd 0.7 Dy 0.3 CrO 3 , Gd 0.5 Dy 0.5 CrO 3 , Gd 0.3 Dy 0.7 CrO 3 , and DyCrO3 samples, respectively. The amount of degradation of XO varied systematically with the dilution of Gd with Dy, illustrating chemical reactivity change brought about by A-site mixing of magnetic ions. To the best of our knowledge, this trend has been reported for the first time in rare-earth orthochromites. Assuming the reaction to follow pseudo-firstorder kinetics, the rate constants for these reactions were estimated to be 4.80 × 10−2, 5.66 × 10−2, 8.62 × 10−2, 9.3 × 10−2, and 11.66 × 10−2 min−1.

3. CONCLUSIONS In summary, the epoxide-mediated gel synthetic approach has been successfully demonstrated for the generation of perovskite-structured GdCrO3, DyCrO3, and the entire set of A-site shared by both the magnetic rare-earth ions from air-stable inorganic salts. The presence of another magnetic ion (next neighbor in the periodic table) at the A-site altered the TN arising from the Cr3+−Cr3+ interaction. The observance of such spin reorientation highlighted the emergence of an interesting magnetic interaction induced by incorporating two different magnetic ions at the A-site. As the optical band gap of GdCrO3 and progressively Dy-substituted samples was in the visible range of the spectrum, they were demonstrated to degrade the aqueous Rh-6G dye solution efficiently in the presence of visible radiation, within which a systematic trend of increased 2662


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10 × 10−6 mol/L at room temperature. Prior to irradiation, the suspension containing the catalyst and the dye solution was stirred in the dark for 40 min so as to attain equilibrium adsorption. After shining the solution with the visible radiation, 5−6 mL of aliquots was taken out from the reaction mixture periodically and centrifuged. Thirty milligrams of the catalyst was added to 20 mL (100 × 10−6 mol/L) of XO solution and 10 mL of H2O2. The concentration of the solutions was determined by measuring the absorbance at λmax = 542 nm (for Rh-6G) and 434 nm (for XO) using a UV−vis spectrophotometer (Shimadzu 1800).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00480. Le Bail refinement of GdCrO3 and progressively Dysubstituted samples, temporal changes in the absorbance spectra of the aqueous Rh-6G dye molecule in the presence of GdCrO3 and progressively Dy-substituted samples and on exposure to visible radiation, and UV−vis absorption spectra of xylenol orange solution in the presence of GdCrO3 and progressively Dy-substituted samples in the presence of H2O2 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (R.N.). ORCID

Rajamani Nagarajan: 0000-0002-0983-7814 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the University of Delhi and DU-DST PURSE for the financial support to carry out this research. V.K.T. thanks the UGC, Government of India for the SRF fellowship. Useful discussions and the usage of DST-funded facilities of Professor S. Uma from the Department of Chemistry, University of Delhi are gratefully acknowledged.

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