MgCr-LDH Nanoplatelets as Effective Oxidation Catalysts for Visible Light-Triggered Rhodamine B Degradation

: In this work, we successfully exfoliated MgCr-(NO 3 − ) LDH with large purity by a simple formamide method followed by post-hydrothermal treatment and characterized by different physico-chemical techniques. The UV-DRS study persuades the red-shifted absorption band and suitable band gap of MgCr-(NO 3 − ) LDH for optimum light harvestation ability related to the optical properties. Alternatively, the production of elevated photocurrent density of MgCr-(NO 3 − ) LDH (3:1, 80 ◦ C) in the anodic direction was veriﬁed by the LSV study, which further revealed their effective charge separation efﬁcacy. These MgCr-LDH nanosheets (3:1, 80 ◦ C) displayed the superior Rhodamine B (RhB) degradation efﬁciency of 95.0% at 0.80 kW/m 2 solar light intensity in 2 h. The tremendous catalytic performances of MgCr-LDH (3:1, 80 ◦ C) were typically linked with the formation of surface-active sites for the charge trapping process due to the presence of uncoordinated metallocenters during the exfoliation process. Furthermore, the maximum amount of the active free atoms at the edges of the hexagonal platelet of MgCr-LDH causes severance of the nanosheets, which generates house of platelets of particle size ~20–50 nm for light harvestation, promoting easy charge separation and catalytic efﬁciency. In addition, radical quenching tests revealed that h + and • OH play as major active species responsible for the RhB degradation.


Introduction
Layered structure material represents an emerging class of two-dimensional (2D) materials that acquire sheet-like morphology with the thickness of single or few-layered atoms [1][2][3]. The importance of layered materials is credited due to their rich interlayer chemistry, such as intercalation and ion exchange properties, which modified their electronic and optical properties. There are lot of many-layered solid materials with few-layer, single-layer, or stacked-layered structures that have been identified as such layered double hydroxide (LDH) [3][4][5][6][7][8][9], layered metal hydroxides [10], layered graphene oxide [4,9], and layered graphitic carbon nitride (g-C 3 N 4 ) [11][12][13][14], in photocatalytic dye degradation and energy conversion reactions. Amongst these, LDHs have been considered as one of the most efficient photocatalysts in producing clean H 2 energy along with environmental abatement following a green technological aspect. Although the direct semiconducting capability of LDH is very much restricted, its lamellar structure smooths the photoinduced electron transfer from bulk to the surface, which is useful in water splitting, and the photogenerated holes are thereof involved in the pollutant degradation [15,16]. Normally, LDHs represents a group of anionic layered materials consisting of positively charged layers with interlayer anions and H 2 O molecules for charge recompense, which is widely used as catalysts, catalyst support, ion exchangers, and electrocatalytic and photocatalytic materials [5]. The generic formula of LDHs is [M(II) 1−x M(III) x (OH) 2 ] x+ [A n− x/n . mH 2 O] x− , where M(II) and M(III) specify the metal atoms and A n− represents the anions, n is the charge upon interlayer anion, m is amount of H 2 O, and x = M III /(M II + M III ). The atomic arrangement broadening of peak at 2θ = 30 • to 60.4 • owing to the presence of formamide and corresponds to the (012) and (110) planes, respectively (Figure 1a (ii)). The absence of (003) and (006) characteristic planes in MgCr (3:1) material reveals the exfoliation of bulk MgCr-LDH. Similarly, MgCr-LDH (2:1) and MgCr-LDH (4:1) follow the similar XRD pattern as that of MgCr-LDH (3:1) with a slight appearance of the (006) plane in MgCr-LDH (4:1). The (006) planes in the PXRD pattern of MgCr-LDH (3:1, 80 • C) at lower 2θ~11-23 • revealed broad and symmetrical basal reflections, while the spiky and asymmetrical reflections of (012) and (110) were expressed at higher 2θ~34-66 • , respectively. The broad (006) and spike (110) planes of MgCr-LDH (3:1, 80 • C) could be correlated with the interlayer height differences and stacking disorder of distinct NS in materials [27,28], whereas the rest of crystalline plane was absence in the PXRD pattern of hydrothermally-treated MgCr-LDH (Figure 1b (v)). The interlayer spacings (d) were deliberate via Braggs law, nλ = 2d sin (θ), in which n = 1, λ is the wavelength of the target and θ is the angle of incidence. The d value of MgCr-LDH (3:1, 80 • C) was found to be 0.46 nm corresponding to d (006) plane and NO 3 − as interlayer anion. Alternatively, the d value of MgCr-LDH (3:1) without hydrothermal was found to be 0.16 nm corresponding to d (110) plane as there were absence of (003) and (006) plane in the PXRD pattern of the material. Hence, hydrothermal temperature played a major role and dramatically altered the crystal growth of the exfoliated MgCr-LDH NS, and the utmost crystallinity was noted at 80 • C for MgCr-LDH (3:1) [29].

Morphological Analysis
TEM images were acquired to trace out the arrangement of the LDH uni/multilamellar NS (Figure 2) [30]. In the moderately dissipate area, the MgCr-LDH lamellae (3:1, 80 • C), possess platelet like shape oriented at random with house-of-card morphology (Figure 2a,b). Contrary to the typical hexagon, at mild hydrothermal temperature of 80 • C, the elevated amount of dynamic in free atoms at the edges of the hexagonal platelet causes intersection of the NS, which generates house of platelets type morphology of MgCr-LDH as shown in Figure 2b. Figure 2c reveals distinct lattice distance~0.38 nm in MgCr-LDH (3:1, 80 • C), which is approximately related to the (006) plane of the 2D MgCr-LDH nanocrystals. The particle diameter of MgCr-LDH (3:1, 80 • C) NS is expected to be average size of 20-50 nm. Furthermore, the morphology of MgCr-LDH (3:1, 80 • C) NS was verified with FESEM analysis. In Figure 2c,d, the morphology of MgCr-LDH (3:1, 80 • C) was composed of irregular matrix and aggregated into larger nanoparticles with rough and porous surface during mild hydrothermal treatment at 80 • C. This might be due to the hydrothermal treatment effect on MgCr-LDH (3:1) materials. Nevertheless, the TEM image of MgCr-LDH (3:1, 80 • C) easily revealed their exact sheet-like morphology of hexagonal nanoplatelets, which are of distinctive features of exfoliated LDH material, and approximately matching with the XRD outcome. Additionally, the EDX analysis of MgCr-LDH (3:1, 80 • C) as shown in Figure 2c confirms that the system contains all the elements like Mg, Cr, and O without any impurity, which proves its compositional purity.
with FESEM analysis. In Figure 2c,d, the morphology of MgCr-LDH (3:1, 80 °C) was com posed of irregular matrix and aggregated into larger nanoparticles with rough and porou surface during mild hydrothermal treatment at 80 °C. This might be due to the hydrother mal treatment effect on MgCr-LDH (3:1) materials. Nevertheless, the TEM image of MgCr LDH (3:1, 80 °C) easily revealed their exact sheet-like morphology of hexagonal nano platelets, which are of distinctive features of exfoliated LDH material, and approximatel matching with the XRD outcome. Additionally, the EDX analysis of MgCr-LDH (3:1, 8 °C) as shown in Figure 2c confirms that the system contains all the elements like Mg, Cr and O without any impurity, which proves its compositional purity.

Optical Study
The electronic and optical characteristic properties of the exfoliated MgCr-LDH (2:1 3:1, 4:1) NS without hydrothermal treatment and MgCr-LDH (3:1) NS with hydrotherma treatment (70, 80, and 90 °C) were determined by UV-Vis DRS spectroscopy techniqu ( Figure 3a). The Figure 3a clearly reveals that each MgCr-LDH photocatalyst possess suf ficient potential to absorb visible light and displayed remarkable increase in the absorp tion intensity, which indicated the enhanced excitonic charge pairs partition efficiency o the resultant materials. In concise, MgCr-LDH (3:1) without hydrothermal displayed broad absorption band within 300-500 nm owing to the ligand to metal charge transfe (LMCT) as O2p → Cr-3d-t2g and absorption band at 500-700 nm is assigned to the 2Eg (D → 2T2g spin allowed transition of Cr 3+ in the tetrahedral coordination sites [9,26]. Specifi cally, the gradual increases in the red shift of the absorption band in MgCr-LDH (3:1) ar owing to the increase in Cr 3+ content in the brucite-like host layers. The exfoliation o MgCr-LDH during mild hydrothermal treatment causes severance and folding of NS with formation of tunnels of hexagonal plates, which acts as light harvestation antenna and triggers electronic transition in the respective orbital of the metallocenters in the catalysts

Optical Study
The electronic and optical characteristic properties of the exfoliated MgCr-LDH (2:1, 3:1, 4:1) NS without hydrothermal treatment and MgCr-LDH (3:1) NS with hydrothermal treatment (70, 80, and 90 • C) were determined by UV-Vis DRS spectroscopy technique ( Figure 3a). The Figure 3a clearly reveals that each MgCr-LDH photocatalyst possess sufficient potential to absorb visible light and displayed remarkable increase in the absorption intensity, which indicated the enhanced excitonic charge pairs partition efficiency of the resultant materials. In concise, MgCr-LDH (3:1) without hydrothermal displayed broad absorption band within 300-500 nm owing to the ligand to metal charge transfer (LMCT) as O2p → Cr-3d-t 2g and absorption band at 500-700 nm is assigned to the 2Eg (D) → 2T 2g spin allowed transition of Cr 3+ in the tetrahedral coordination sites [9,26]. Specifically, the gradual increases in the red shift of the absorption band in MgCr-LDH (3:1) are owing to the increase in Cr 3+ content in the brucite-like host layers. The exfoliation of MgCr-LDH during mild hydrothermal treatment causes severance and folding of NS with formation of tunnels of hexagonal plates, which acts as light harvestation antenna and triggers electronic transition in the respective orbital of the metallocenters in the catalysts. Importantly, the absorption edge of MgCr-LDH (3:1, 80 • C) moved towards a longer wavelength with broad and intense absorption from 400 to 800 nm, which is due to the reduced thickness of the exposed atomic sites of the nanolayers that minimized the charge transfer distance and endorsed for effective compilation of charge concentrated over the conductive NS. Furthermore, the inimitable structure in MgCr-LDH (3:1, 80 • C) endorsed light to scatter frequently inside the structure to increase the optical distance and enhance light absorption capacity. In addition, the atomically reduced thickness of the exposed atomic sites of the uni/multi-nanolayers of MgCr-LDH (3:1, 80 • C) NS is another factor responsible for the broad and intense visible light absorption capability within 400-800 nm.
light to scatter frequently inside the structure to increase the optical distance and enhance light absorption capacity. In addition, the atomically reduced thickness of the exposed atomic sites of the uni/multi-nanolayers of MgCr-LDH (3:1, 80 °C) NS is another factor responsible for the broad and intense visible light absorption capability within 400-800 nm.
(αhν) 1/n = A(hν−Eg) (1) where ν and α are the light frequency and adsorption coefficient, respectively Figure 3b shows two types of band-gap energy resides in MgCr-LDH (3:1, 80 °C), which is of 2.54 (Eg1) and 3.98 eV (Eg2) due to the presence of LDH phase, and falls under directly allowed transition. In addition, the Eg1 can also be assigned to the direct electronic transition from O2p to Crnd levels of MgCr-LDH [12]. Furthermore, the Eg2 of MgCr-LDH samples can be assigned to the existence of electronic transition from O2p to Mgns/np [12].

FTIR Study
FTIR plot of MgCr-LDH (3:1, 80 °C) is represented in Figure 4 [31]. The sample displayed a broad absorption band at around 3374 cm −1 , which signifies the occurrence of -OH group of H2O molecules [32]. Similarly, the two distinct bands at 1639 and 1440 cm −1 , corresponded to the stretching mode of vibration in Mg-O and bending mode of vibration in adsorbed H2O over MgCr-LDH surface [32]. (αhν) 1 where ν and α are the light frequency and adsorption coefficient, respectively Figure 3b shows two types of band-gap energy resides in MgCr-LDH (3:1, 80 • C), which is of 2.54 (E g 1) and 3.98 eV (E g 2) due to the presence of LDH phase, and falls under directly allowed transition. In addition, the E g 1 can also be assigned to the direct electronic transition from O2p to Crnd levels of MgCr-LDH [12]. Furthermore, the E g 2 of MgCr-LDH samples can be assigned to the existence of electronic transition from O2p to Mgns/np [12].

FTIR Study
FTIR plot of MgCr-LDH (3:1, 80 • C) is represented in Figure 4 [31]. The sample displayed a broad absorption band at around 3374 cm −1 , which signifies the occurrence of -OH group of H 2 O molecules [32]. Similarly, the two distinct bands at 1639 and 1440 cm

Electrochemical Study
LSV study of MgCr-LDH (3:1, 80 °C) samples was conducted in a potential panel o −1.0 to 1.2 V, using 0.1 M Na2SO4, and scan rate of 10 mV·s −1 to reveal the photocurren retaliation of the as-synthesized catalysts. Figure 5 exposes the photocurrent capacity measures under dark and light environments. As shown in Figure 5, MgCr-LDH (3:1, 80 °C) could be able to produce current density of 1.20 µA/cm −2 under light exposure. Th progress of the photocurrent in the anodic direction shows that all the MgCr-LDH sam ples filling the properties of n-type semiconductor [11]. The oxidation peak intensity de creases gradually at optimal loading density of Cr 3+ in MgCr-LDH (3:1) and further mild hydrothermal treatment at 80 °C provides compact NS structure of MgCr-LDH, where th oxidation peak of Cr → Cr 3+ decreases gradually and disappear, suggesting the enhanc stability of MgCr-LDH (3:1, 80 °C) in the nanostructure. The enhanced stability of MgCr LDH (3:1, 80 °C) causes superior visible light driven photocatalytic activity. The flat band potential of the entire MgCr-LDH sample was detected at −0.60 V vs. Ag/AgCl, pH = 6.5 The flat band potential is directly correlated to baseline of the conduction band (CB) of an n-type semiconductor [11]. As 2.54 eV is the primary Eg of MgCr-LDH, the corresponding valence band maximum (VB) was +1.94 V. Particularly, electrode potential was trans formed to NHE by the subsequent Equation (2) [34]: Hence, CB and VB of MgCr-LDH in NHE scale were determined to be −0.01 and +2.53 V. Similarly, the dark current measurement of MgCr-LDH (3:1, 80 °C) showed a sligh incremental current density as compared to light current density.

Electrochemical Study
LSV study of MgCr-LDH (3:1, 80 • C) samples was conducted in a potential panel of −1.0 to 1.2 V, using 0.1 M Na 2 SO 4 , and scan rate of 10 mV·s −1 to reveal the photocurrent retaliation of the as-synthesized catalysts. Figure 5 exposes the photocurrent capacity measures under dark and light environments. As shown in Figure 5, MgCr-LDH (3:1, 80 • C) could be able to produce current density of 1.20 µA/cm −2 under light exposure. The progress of the photocurrent in the anodic direction shows that all the MgCr-LDH samples filling the properties of n-type semiconductor [11]. The oxidation peak intensity decreases gradually at optimal loading density of Cr 3+ in MgCr-LDH (3:1) and further mild hydrothermal treatment at 80 • C provides compact NS structure of MgCr-LDH, where the oxidation peak of Cr → Cr 3+ decreases gradually and disappear, suggesting the enhance stability of MgCr-LDH (3:1, 80 • C) in the nanostructure. The enhanced stability of MgCr-LDH (3:1, 80 • C) causes superior visible light driven photocatalytic activity. The flat band potential of the entire MgCr-LDH sample was detected at −0.60 V vs. Ag/AgCl, pH = 6.5. The flat band potential is directly correlated to baseline of the conduction band (CB) of an n-type semiconductor [11]. As 2.54 eV is the primary E g of MgCr-LDH, the corresponding valence band maximum (VB) was +1.94 V. Particularly, electrode potential was transformed to NHE by the subsequent Equation (2) [34]: Hence, CB and VB of MgCr-LDH in NHE scale were determined to be −0.01 and +2.53 V. Similarly, the dark current measurement of MgCr-LDH (3:1, 80 • C) showed a slight incremental current density as compared to light current density.

Photocatalytic RhB Degradation Activity
The RhB degradation activity of MgCr-LDH samples was studied under solar light exposure. Self-degradation study of RhB was performed by exposing under the solar energy for 30 min, and the results showed that the RhB self-degradation was roughly insignificant. Furthermore, the adsorption study of the catalyst was performed under dark condition for 30 min. The RhB degradation study was activated by dispersion of 0.03 g of the catalyst in 20 ppm of RhB (20 mL) under solar energy for 120 min. The exfoliation of MgCr-LDH under mild hydrothermal condition generates uncoordinated metallocenters and dense amount of free atoms at the edges of hexagonal platelet responsible for oxygen related vacancies and causes intersection of the NS for enhancing light harvestation ability of the materials and corresponding exciton separation efficiency directly or indirectly responsible for the photooxidation of RhB to non-toxic products. The RhB photodegradation activities of all of the as-synthesized MgCr-LDH samples were measured in accordance with the following Equations (3)

Photocatalytic RhB Degradation Activity
The RhB degradation activity of MgCr-LDH samples was studied under solar light exposure. Self-degradation study of RhB was performed by exposing under the solar energy for 30 min, and the results showed that the RhB self-degradation was roughly insignificant. Furthermore, the adsorption study of the catalyst was performed under dark condition for 30 min. The RhB degradation study was activated by dispersion of 0.03 g of the catalyst in 20 ppm of RhB (20 mL) under solar energy for 120 min. The exfoliation of MgCr-LDH under mild hydrothermal condition generates uncoordinated metallocenters and dense amount of free atoms at the edges of hexagonal platelet responsible for oxygen related vacancies and causes intersection of the NS for enhancing light harvestation ability of the materials and corresponding exciton separation efficiency directly or indirectly responsible for the photooxidation of RhB to non-toxic products. The RhB photodegradation activities of all of the as-synthesized MgCr-LDH samples were measured in accordance with the following Equation (3): The order of intensification of RhB degradation for series of MgCr-LDH was 75% (MgCr 4:1), 85% (MgCr 2:1), 90% (MgCr 3:1), 93% (MgCr 3:1, 70 • C), 95% (MgCr 3:1, 80 • C), and 91% (MgCr 3:1, 90 • C), respectively (Figure 6a). These outcomes evidently show that (MgCr 3:1, 80 • C) exhibits enhanced RhB degradation. In addition, an excess substitution of Cr 3+ to Mg 2+ (MgCr 4:1) results decreases in activity because of the blocking of the reactive phases of MgCr-LDH. The rate of RhB degradation and related spectral changes of absorbance are depicted in Figure 6b. MgCr (3:1, 80 • C) shows excessive potential for maintaining higher stability approximately to the extent of 3 cycles (Figure 6c) than other as-prepared materials as discussed so far. The degradation activity results are in fine matching with the characterization results.

Kinetics of the RhB Degradation
The kinetics of RhB degradation follows pseudo-first-order and the experimental data were fitted to the Langmuir−Hinshelwood kinetic model using the following Equations (4) and plotted in Figure 7a: The k denoted as the apparent rate constant. The rate constant (k) of the RhB degradation reaction for different MgCr-LDH samples was calculated by linear fitting of the ln (C0/C) vs. time plot (Figure 7a) and consequently, the measured slope of ln (C0/C) vs. time plot provides the value of k. The k value depicted in Table 1 clearly shows that RhB degradation for MgCr-LDH based samples follows pseudo-first order kinetics in Langmuir−Hinshelwood model. The fitted line parameters related to the regression coefficient (R 2 ) are also given in Table 1.

Kinetics of the RhB Degradation
The kinetics of RhB degradation follows pseudo-first-order and the experimental data were fitted to the Langmuir−Hinshelwood kinetic model using the following Equations (4) and plotted in Figure 7a: The k denoted as the apparent rate constant. The rate constant (k) of the RhB degradation reaction for different MgCr-LDH samples was calculated by linear fitting of the ln (C 0 /C) vs. time plot (Figure 7a) and consequently, the measured slope of ln (C 0 /C) vs. time plot provides the value of k. The k value depicted in Table 1 clearly shows that RhB degradation for MgCr-LDH based samples follows pseudo-first order kinetics in Langmuir−Hinshelwood model. The fitted line parameters related to the regression coefficient (R 2 ) are also given in Table 1.

Scavenger Study for the Radicals
The scavengers taken active part in the RhB degradation study was examined by distinct trapping reagents as para-benzoquinone (p-BQ), dimethyl sulfoxide (DMSO), isopropanol (IPA), and ethylenediaminetetraacetic acid (EDTA) for scavenging superoxide

Scavenger Study for the Radicals
The scavengers taken active part in the RhB degradation study was examined by distinct trapping reagents as para-benzoquinone (p-BQ), dimethyl sulfoxide (DMSO), isopropanol (IPA), and ethylenediaminetetraacetic acid (EDTA) for scavenging superoxide (•O 2 − ), electron (e − ), hydroxyl (•OH), and hole (h + ) radicals, sequentially. In the experiment process, 5 mM of each trapping agent was incorporated to 20 mL of 20 ppm RhB added with 0.03 g of catalyst and set for degradation reactions. The scavenger test results shows an increased rate of RhB degradation (15,35,55, 70%) with addition of EDTA, IPA, p-BQ, and DMSO scavenging reagents, as depicted in Figure 7b. This result shows the participation of hole and hydroxyl radicals as primary and superoxide as secondary active species for RhB degradation process.

Confirmatory Test for •O 2 − Radicals
The •O 2 − radical was confirmed via a nitroblue tetrazolium (NBT) test [35]. Then, NBT was used to determine dense of •O 2 − and disclose the photodegradation efficiency of MgCr-LDH. Figure 7c shows no such extent of variation in the NBT concentration by MgCr-LDH, before and after the RhB photodegradation reactions, which reveals that CB potential of MgCr-LDH is not sufficient to directly produce •O 2 − radicals.

Confirmatory Test for •OH Radicals
Terephthalic acid (TA) PL probe was utilized for the detection of •OH radical [36]. TA directly reacts with the •OH radical, producing 2-hydroxyterephthalic acid (TAOH) by emission band at 426 nm for excitation at 315 nm. Then, a~0.02-g catalyst was supplied to 0.004 M of NaOH solution consisting of 20 mL of TA, and subsequently the suspension was manifest to solar energy for 2 h. Afterward, intense PL peak of TAOH solution quantify the •OH radical formed through photooxidation reactions. The maximum intensity of TAOH peak extends the highest percentage of •OH formation. Figure 7d shows the formation of primary •OH radicals in RhB degradation catalyzed by MgCr-LDH.

Mechanism of RhB Degradation by MgCr-LDH
The mechanistic path of RhB degradation by using MgCr-LDH catalyst (Figure 8), could be clearly elucidated in terms of structural and morphological features, together with band gap sequence, and active sites, which correlated to the formation of exfoliated NS with photoinduced carrier charge separation for enhanced activity. The apparent enhancement in the photocatalytic RhB degradation over MgCr-LDH is owing to the formation of constant layer charge density, morphological aspects by mild hydrothermal temperature, and surface-active sites during the exfoliation process. Normally, the exfoliation rate decreases at a lower x = 0.2, due to tight interlayer gallery for which the polar interactions towards formamide are reasonable owing to low layer charge density. Though Columbic/electrostatic interactions are considered as an extraneous parameter in LDH containing divalent anion that limits the rate of intercalation, but we noticed the well exfoliation of MgCr-(NO 3 − ) LDH at x = 0.3, and provided maximum space for formamide intercalation and consequently increases the basal spacings. It is further noted that high layer charge density could certainly result in strong interaction with formamide, and in this energy balance process, the electrostatic interactions may stabilize the LDH compound. Further MgCr-(NO 3 − ) LDH at x = 0.3 provided platelets such as morphology as revealed from TEM image. It is also expected that the degree of exfoliation of Mg II 0.9 Cr III 0.3 (OH) 2 (NO 3 ) 0.3 . nH 2 O reached to approximately complete exfoliation by post-hydrothermal treatment [13]. Further XRD also revealed a clear node of (006) plane considerably on hydrothermal post-treatment, in context to Cr(III) proportion. This shows a homogeneous allocation of H 2 O molecules and out of plane NO 3 − ion orientation and causes interlace crystallites. The LSV analysis also confirmed that MgCr-LDH (3:1, 80 • C) possess an intrinsic n-type characteristic properties of materials. By utilizing the Tauc plot ( Figure 3b [15,37]. Alternatively, the depth VB position of MgCr-LDH (+2.53 V vs. NHE) was relatively sufficient for the direct oxidation reaction of holes with adsorbed H 2 O molecules to form •OH radicals E Θ (•OH/OH − = +1.99 eV vs. NHE) [38]. These concepts were verified by the scavenger test, where h + were of the primary active-species in MgCr-LDH, which were responsible for the photodegradation of RhB. Consequently, these •OH radicals reacted with RhB over the surface of MgCr-LDH (3:1, 80 • C) to produce non-toxic products.

Photocatalytic RhB Degradation Activity
(Conditions: exfoliated MgCr-LDH = 0.03 g, [RhB] = 20 ppm, exposer time = 120 min). The photodegradation of RhB was executed in batch mode using a 20-ppm-concentrated RhB aqueous solution and 0.03 g of the catalyst. The suspension was uncovered to sunlight (~solar intensity = 0.80 kW/m 2 ) in closed Pyrex conical flasks with steady stirring during hot summer days. Prior to the solar experiments, dark reactions were carried out for comparison. The RhB degradation was analyzed by the spectrophotometric technique at 554 nm. After 2 h of solar irradiation, the conversion was reached up to 95.0%. Furthermore, the stability test of the catalyst for RhB degradation was repeated for three cycles. The stability test of the photocatalyst was executed in each run by simply washing with ethanol and deionized water followed by oven-drying at 80 • C for next use in second cycle.

Materials Characterization
Powder X-ray diffraction (PXRD) was performed by a Rigaku Miniflex powder diffraction meter, using Cu Kα source (λ = 1.54 Å, 30 kV, 50 mA). The bending and stretching modes of vibration of the materials were carried out by JASCO Fourier transform infrared (FTIR)-4600, using the KBr reference. The ultraviolet-visible diffuse reflectance spectra (UV−Vis DRS) were produced by a JASCO-V-750 UV−Vis spectrophotometer using BaSO 4 as a reference. Photoluminescence (PL) was analyzed using an FP-8300 JASCO spectrofluorometer. A high-resolution transmission electron microscopy (HR-TEM) and energy dispersive X-ray (EDX) study was carried by JEM-2100F at an accelerating voltage of 200 kV. The field emission scanning electron microscopy (FESEM) micrograph was acquired by a HITACHI 3400N microscope. The entire photoelectrochemical (PEC) measurements were carried out by potentiostat−galvanostat (IVIUM n STAT multi-channel electrochemical analyzer), with accessories of a 300 W Xenon lamp for visible light supply, a three-electrode system containing Pt, Ag/AgCl, and fluorine-doped tin oxide (FTO), as a counter, reference, and working electrode, respectively. The working electrode was made by an electrophoretic deposition process by our earlier reported method [15]. The electrolyte contained 0.1 M of Na 2 SO 4 solution. The linear sweep voltammetry (LSV) study was completed by applied bias within −1.0 to +1.2 V at scanning rate of 5 mV s −1 in visible light exposure.

Conclusions
In summary, our thorough investigations on MgCr-(NO 3 − ) LDH disclosed the exfoliation capability of MgCr-(NO 3 − ) LDH in formamide into uni/multi-layer NS, which strongly depended on layer charge (Mg 2+ /Cr 3+ = 2:1, 3:1, and 4:1) and hydrothermal temperature (70, 80, and 90 • C). With an optimum metallic (Mg 2+ /Cr 3+ ) ratio of 3:1, and hydrothermal treatment of 80 • C for 24 h, the exfoliated MgCr-(NO 3 − ) LDH, displayed superior photocatalytic RhB degradation (95.0%) under solar light exposure for 2 h. This synthetic process is simple, cost-effective and thus potential strategy for the production of stable and exfoliated MgCr-LDH into uni/multi-layer NS. Finally, the morphology is a vital aspect in determining the nanosheet structure and by applying an optimized synthetic protocol; we tried to maintain house-of-nano-platelet morphology in MgCr-(NO 3 − ) LDH (3:1, 80 • C) for light absorption, fast charge separation, and transfer for superior catalytic activities. Consequently, post-hydrothermal treatment might be suitable to augment productivity in exfoliation procedure. This work is an effectual approach to optimize novel catalytic system with high efficiency and stability, but initiate new ground for the vast application of MgCr-LDH in environmental remediation as well as energy production. Institutional Review Board Statement: Ethical review and approval were waived for this study.

Informed Consent Statement: Not applicable.
Data Availability Statement: Data sharing is not applicable to this article.