Superlative Photoelectrochemical Properties of 3D MgCr- LDH Nanoowers inuencing towards Photoinduced Water Splitting Reactions

Layered double hydroxides (LDHs) are competent photocatalysts for water splitting reactions, vital to produce solar fuels, but their restricted available reactive sites, slow mass and charge transfer, are yet remain a challenge. To surmount these lacunas, Nanoowers-like three-dimensional (3D) open structure of MgCr-LDH have been designed in a substrate-free path by one-step formamide assisted hydrothermal treatment followed by visible light irradiation and utilized as ecient photocatalysts for the H 2 and O 2 production. The structural, morphological, optical and photoelectrochemical (PEC) properties of the MgCr-LDH nanoowers were extensively examined, by various physico-chemical characterization techniques. Moreover, the well-designed 3D MgCr-LDH nanoowers with open structure were formed by a stacking of numerous 2D nanosheets, which inherently triggered with magnicent PEC properties, including high current density of 6.9 mA/cm 2 , smallest arc of the Nyquist plot (59.1 Ω cm −2 ) with photostability of 6000 s thereby enhancing the photocatalytic water splitting activity along. Moreover such a perfectly self-stacked 2D nanosheet in 3D MgCr-LDH possess defect sites as enriched 50% oxygen vacancy resulting a good contact surface within the structure for effective light absorption and easy electron and hole separation, facilitates the adsorption of protons and intermediate of water oxidation. Further, the doped Cr 3+ pull up electrons from water oxidation intermediates, thereby displaying superior photocatalytic H 2 and O 2 production activity of 1315 µmol/h and 579 µmol/h, respectively. Favorable oxygen vacancy type defect surface with Cr 3+ dopant in MgCr-LDH triggers signicant PEC properties, which inuences the easy charge transfer and separation mechanism and robustly enhance the photocatalytic performance of the nanoower. as-synthesized possible formation of •OH of the terephthalic acid The •OH formation ability of the MgCr-LDH/NF could as the effective separation of pairs via appropriate amount of oxygen vacancies and Cr 3+ dopant for enhancing the kinetics of water oxidation of holes in the VB of the concerned material. the calculated VB potential of MgCr-LDH/NF is NHE, vs. − •OH radical photocatalytic water splitting


Introduction
In progress of time, the rapid and massive exhaustion of traditional fuels, such as crude petroleum oil, coal, and natural gas, accelerates the high demand in the advancement of sustainable energy resources to meet up the adequate requirements 1 . And it is quite imperative to enlarge the green and clean energy sources such as solar energy, H 2 energy, hydrothermal energy, wind energy, tidal energy, and geothermal energy, etc., to lessen the environmental majors [2][3][4] . Opportunely, photocatalytic (PC) or PEC water splitting to produce H 2 and O 2 is considered as one of the most super uities of green technological approaches to indulgence the solar-to-chemical energy conversion for addressing the worldwide energy shortage 5,6 . The hydrogen evolution reaction (HER), and oxygen evolution reaction (OER), has been investigated for decades and regarded as a vital energy conversion reaction, as in water electrolysers 7,8 .
these aspects, combination of Mg 2+ cation with insertion of Cr 3+ cation as dopant in MgCr-LDHs are signi cant as Cr 3+ ions in partial substitution to the octahedral sites of Mg 2+ cation layers could behave as reactive sites to promote water splitting, while Mg ions offer structural stability of LDHs. In addition, the incorporation of doped Cr 3+ metal cation into the lattice of MgCr-LDH hold an electronic con guration of t 2g 3 e g 0 , in which the vacant e g orbital could be favorable for the capture of electron from the defect sites and corresponding carrier charge transfer process stabilized the system by enhancing the kinetics of water splitting reactions 63 .
Motivated by the promising properties of exfoliated LDH nanosheets, in this context, we fabricated 3D hierarchal binary MgCr-LDH nano ower assembled with 2D nanosheets and oxygen vacancy defect sites by a mild hydrothermal strategy followed by visible light irradiation, which established these materials as a highly active photocatalytic water splitting catalyst with enhanced photoelectrochemical properties for future PEC photoanode materials. To the best of our knowledge, these 3D MgCr-LDHs as model systems possessing excellent PEC properties have not conducted earlier concerning photocatalytic water splitting reactions. These kind of 3D binary MgCr-LDH nano ower assembled with 2D nanosheets and oxygen vacancies as defect sites provides many advantages in variety of ways: (i) superior electronic transportation, (ii) augmentation of the synergic effects amongst Mg, and Cr, and (iii) presence of Cr +3 as dopant, behaves as a pool of electrons by pulling electrons from the oxygen vacancies which used for trapping of electrons, thus swiftly regenerate the active sites for effective water splitting reactions, (iv) As is known, Mg(OH) 2 is almost inactive under visible light and causes negligible carrier charge excitation through water splitting process and from an elemental perspective, the Cr 3+ cation presents in the MgCr-LDH reveals its special electronic con guration of vacant e g orbitals, which facilitates the electronic charge transfer process, thus anticipated to augment the conductivity and certainly promote the boosting of reaction performances and reusability of the photocatalyst. Moreover, the fabricated bimetallic MgCr-LDH acts as a versatile solitary photocatalyst, by providing depth insight into the role of oxygen vacancies type defect sites, doped Cr 3+ ions and open 3D ower like structure for enhancing PEC properties together with promoting charge transfer process for photocatalytic water splitting performances of the LDH layer, whose mechanistic insight has been discussed in detail.

Corroboration of Perception
Engineering of the morphological features to refrain the existing active sites with creation of new defect sites plays an utmost vital role for an effective excitonic partition and electronic channelization in light driven catalytic reactions 64 . Apart this, the development of green and cost-effective photocatalytic system in terms of substrate-free particulate 3D binary MgCr-LDH owers via a combination of simplistic hydrothermal technique followed by visible light illumination could be regarded as a novel approach towards sustainable energy utilization 65 . This type of 3D binary MgCr-LDH nano ower assimilated by 2D nanosheets propose assured advantages without complex pre/post-treatments together with an effective amalgamation of pre-existing active sites of Cr(OH) 3 and oxygen related defect sites for effective electronic transportation resulting out magni cent PEC properties towards photoinduced water splitting reactions 66, 67 . This resourceful practice certi es single-step synthesis of colloidal MgCr-LDH NSs and thanks to the oxygen vacancies on the MgCr-LDH NSs which mostly provided active sites for further nucleation and crystallization process. The growth process of the 3D MgCr-LDH nano ower structures could be described as follows ( Figure 1); a signi cant and time-saving methodology has been adopted to deliver the signi cant structural transformation of exfoliated MgCr-LDH NSs to hierarchal 3D structure of MgCr-LDH matrix. Firstly, the well-controlled growth of MgCr-LDH NSs from MgCr-LDH PSs was accomplish by the use of hydrolyzing agent HCHO 68 , together with OH¯ following a coprecipitation and dispersion by sonication process 69

Morphological Features Analysis
The eld emission scanning electron microscopy (FESEM) techniques were used to reveal the morphologies of the as-synthesized MgCr-LDH/NF. Figure 2 (Figure 2(a)) on the previously formed layers (Figure 2(b)). As discussed, the layered 2D MgCr-LDH NSs interconnected to create 3D nano owers consisting of 2D NSs with an open structure; besides, these kind of morphological aspects furnish an enormous amount of available surface, which manifest enrich photo/electroactive sites for the water redox reaction, and open space for ion pooling for escalating the kinetics of diffusion barrior within the electrode/electrolyte interface 71 .
Following the FESEM analysis, the structural aspects of the 3D MgCr-LDH nano owers, could be wellrecognized vide transmission electron microscopy (TEM) and high resolution-TEM (HR-TEM) analyses. TEM images of MgCr-LDH NS ( Figure ), and MgCr-LDH/NF (Figure 2(c)) elucidate the effect of HCHO induced mild hydrothermal treatment and visible light irradiation on structures and morphologies of materials. Figure 2(c-e) exempli ed the distinct and uffy nature of the characteristic 3D MgCr-LDH materials. Further the TEM image also illustrates the consistency of dense and thin 2D nanosheets ( Figure S1), in typical 3D MgCr-LDH nano ower 72 . The free and exposed 2D NSs surface ease out catalyst reactions and triggers the photocatalytic water splitting activities of binary MgCr-LDHs 73 . Furthermore, the obscure part appeared owing to the dense stacking, and distortation of the NSs and these properties could also be identi ed in graphene and analogus materials 74 . The high resolutiontransmission electron microscopy (HR-TEM) images of MgCr-LDH/NF (Figure 2(f)) offer a distinct view of lattice distance ~0.26 nm, represented by dotted lines, which is approximately matching with the typical (012) plane in 2D MgCr-LDH NSs. The particle diameter of MgCr-LDH/NF is assumed to be average distance of 20-50 nm. A similar morphological pattern is also detected in NiAl LDHs, 47 etc. The inset selected area electron diffraction (SAED) pattern ( Figure 2(f)) also con rms the (003), and (012) planes of the LDH fully matching with the X-ray diffraction (XRD) pattern ( Figure 3). These results signify the polycrystalline nature of the NSs in the owery like binary LDHs 47 . Furthermore, sharp contrast elemental mapping of the Mg, Cr, and O together with the energy dispersive X-ray spectroscopy analyses (EDX) spectral plot clearly specify the uniform allocation of constituent elements in MgCr-LDH/NF ( Figure S2 (ad)).
Structural and valence state features of binary MgCr-LDHs (3:1) The solid state crystallographic planes of MgCr-LDHs (3:1) based samples were characterized through powder XRD patterns and the entire diffraction pattern could be resemble into a hexagonal crystal phase with a space group R3m and rhombohedral symmetry of hydrotalcite like materials ( Figure 3). The diffraction pattern of MgCr-LDH/PS (Figure 3(a)), consisting of three main peaks at 2 = 10°, 19.2°, and 34.5° 56.2 ascribed to the phase re ection of the (003) and edge plane of (012) along with the peak at is indexed to the (110) edge plane 75 . These (012) and (110) edge planes in XRD pattern of LDH are previously considered as the main exposed planes of LDHs and match up to the cationic and anionic distances within the layered structure. The peak index of the (110) re ection approximately at 2 = 56. MgCr-LDH/NF (Figure 3(b)), exhibits sharp and broad re ection planes of the main planes of (003), (012) and (110) at 2 = 12.6°, 35.4° and 61.2°, respectively. The relatively shifting of the intense and broad re ections peaks of MgCr-LDH/NF to higher 2 angle quite indicative of the decrease in the interlayer distance, which is an indicative of the assembling of the nanosheets and corresponding evolution of the ower like structure. This consequences are further veri ed by the decrease in interlayer distance of 1.5110 Ȧ relative to the (110) basal planes. Furthermore, the missing of the (006) basal re ection planes demonstrate a reduction in periodicity of basal re ection plane owing to the association of nanosheets in nano owers. This implies that the crystal sizes are reduced in both lattice parameter a (a = 2d(110)) and c (c = 3d(003)) directions, indicative of self-stacking thickness of LDH nanosheets in nano owers. The variations in crystallographic information of MgCr-LDH based samples are included in Table S1.
The Fourier transform infrared (FT-IR) spectroscopy ( Figure S3) also explicates the alteration of molecular units during the formation of MgCr-LDH/NF. In the matter of MgCr-LDH/NS, the broad shoulder band identi ed at 3800 cm −1 and 2600 cm −1 was merged into an intense and weak broad band approximately at about 3500 cm −1 and 3000 cm −1 , respectively 77  photocatalytic water oxidation performances. The Tafel slope is mostly utilized to authenticate the superior OER properties of various binary LDH, which is considered as the rate determining step in the water splitting process; and is deliberate by below equation 82 : where , a, b, and j correspond to the overpotential, constant, Tafel slope, and current density. The Tafel slopes were determined from LSV plots by plotting V versus logj and calculated to be 239, 192, and 82 mV/decade for MgCr-LDH/PS, MgCr-LDH/NS, and MgCr-LDH/NF, respectively ( Figure 5(c)). It was found that morphological variation from bulk to nanosheets and further nano ower like assembly decreases the Tafel slopes and the smallest slope was tenable for MgCr-LDH/NF, con rming the highest current density and faster kinetics towards water splitting reactions. Normally, lower overpotential and smaller Tafel Importantly, the at band potential of MgCr-LDH/NF indicates a decrease in band bending and higher slope assigned to the increased in carrier density, which is attributed to the defect-sites allowed to the charge transfer process among the electrode and electrolyte. Hence, the signi cant charge transfer rate in AgNO 3 as sacri cial agents. In Figure 8(a), it is found that as the structural transformation increases from the bulk phase to nanosheets and gradually increases towards nano ower, the H 2 and O 2 production shows a volcanic trend. The enhanced water splitting activity of binary MgCr-LDH/NF might be owing to the distinctive structural features (3D owers stacked with 2D nanosheets) and the synergistic effects among the dispersion of Mg, and Cr atoms as found from TEM results. Figure 8( 47 . This is also re ected in the XPS spectra and impedance plots of the magni cent PEC properties, and the formation of ower structure is more conducive to H 2 production because of the special structure of the layered 2D nanosheets inside the 3D ower offers added active phases, which amplify the excitonic separation process, so facilitates quick redox reaction.  (Figure 8(c, d)). Each cycle experiment is 125 mins, and a total of 4 cycles are performed. In the 3rd and 4th cycle, the H 2 and O 2 production gradually decreases due to the consumption of sacri cial reagents. The H 2 and O 2 evolution shows that the MgCr-LDH/NF photocatalyst has good catalytic stability. In addition, XRD patterns were executed on the catalysts before and after the cycle of hydrogen production, as shown in Figure S6. It was found that the XRD patterns of the catalyst before and after the cycle of hydrogen production did not change signi cantly, except a little reduction in peak intensity which may be due to loss in catalyst handling, surface blocking by the sacri cial reagents and may be corrosion of catalysts surface during the catalytic reaction. These features indicate that the MgCr-LDH/NF catalyst has excellent water splitting activity.
Additionally, the H 2 production experiment of MgCr-LDH/NF was carried out under the presence of different sacri cial agent (10% lactic acid solution, 10%methanol, 10% triethanol amine (TEOA)) under similar experimental condition as shown in Figure 9 (a). The sacri cial based water splitting reaction depends upon various factors such as the oxidation potential of the reagent, polarity, chain length, sideproduct formation, adsorption on catalyst surface, number of hydroxyl groups etc. Experiments show that the highest hydrogen production is in the 10% CH 3 OH aqueous solution. This is because of the easy electron donor in the reaction system, and more electrons are generated and transferred to the active part of the photocatalyst for H 2 generation reaction, further the reagent oxidized by photogenerated holes in the VB of LDH to CO 2 . The detailed of the mechanism are as predicted in the following equations. Further, scavenger experiment was performed to trace out the active species responsible for water oxidation by using different sacri cial agents such as AgNO 3 , isopropyl alcohol (IPA), ethylenediamine tetraacetic acid (EDTA-2Na) as displayed in Figure 9(b). It was pragmatic that the O 2 formation activity is maximum in case of AgNO 3 , whereas on addition of IPA, and EDTA-2Na (hole scavenger), the reduction performance increases which indicates the active role of hole in the water oxidation process. Yet again, for quantifying the e ciency of the photocatalyst towards O 2 production, the apparent conversion e ciency, was measured to be for photocatalytic O 2 evolution by MgCr-LDH/NF system under visible light irradiation. Considering this results, the •OH radical formation was experimented over different assynthesized samples (MgCr-LDH/PS, MgCr-LDH/NS, and MgCr-LDH/NF) and the results depict the highest possible formation of •OH radical, signifying the most resolute photoluminescence (PL) peak of the terephthalic acid (TA)-OH complex over MgCr-LDH/NF as shown in Figure 9 (c).  (23) Insight into the possible photocatalytic mechanism of charges separation  and oxygen vacancies peaks of MgCr-LDH/NF was identi ed at 500 nm and 524 nm, respectively, which was due to the occupancy of the numerous folded nanosheets during the secondary growth period of nano ower structure to reduce their surface energy, and release of the strong stress, under exterior forces for instance electrostatic, van der Waals forces, and hydrogen bonds in which twisted nanosheets selfassembled into stable and irregular 3D nanostructures 85 . MgCr-LDH/PS displays the strongest PL peak signifying higher e ciency of excitonic recombination process. The most diminished PL peak of MgCr-LDH/NF at about 373-500 nm reveals the lower recombination rate of photoinduced excitonic pairs.
Hence, the suppression in excitonic charge pairs in MgCr-LDH/NF is associated with electron and hole trapping sites, which increases the fate of electronic charge pairs for trigging superior water splitting performances. Generally, the smaller the impedance arc radius, the faster the charge carriers separation.
The radius corresponding to the above sample Nyquist circle is MgCr-LDH/PS> MgCr-LDH/NS> MgCr-LDH-NF. In summary, the MgCr-LDH-NF combination can not only use the internal oxygen vacancy and Cr 3+ dopant as barrior for the electron-hole recombination to accelerate the separation of carriers, but also build an effective electron transfer channel, accelerate electron transfer, and improve the charge trapping ability.
In general, photocurrent response is used to reveal the phenomenon of photogenerated electrons generated by photoexcitation of photocatalyst. As we all know, the higher the photocurrent response With these valid discussions, the possible CB and VB position of MgCr-LDH/NF and the mechanism of water reduction and oxidation reaction over MgCr-LDH/NF were proposed in Figure 12. With the visible light irradiation, semiconductors could absorbed photon energy equal to or greater than the band gap energy, and get excited to produce electrons and hole pairs. The photogenerated electrons transition from the VB position of MgCr-LDH/NF to the CB, and leaving behind holes in the VB. The electrons accumulated on the CB of MgCr-LDH/NF are easily trapped by the Ov center together with the Cr 3+ cations presents unique electronic arrangement (t 3 2g e 0 g ), which facilitate electron capture to reduce the H + in the solution to facilitate H 2 production (H + /H 2 (0 vs. RHE), whereas the holes are consumed by the sacri cial agents 38 Table S2 and Table S3 in Figure 13. Hence, the entire MgCr-LDH/NF can be regarded as a high-e ciency PEC cell assembly connected in three electrode series. This is advantageous to the improvement of hydrogen evolution performance.. In addition, compared with other variant of LDH-based photoelectrode, the MgCr-LDH/NF photoelectrode also reveals comparable PEC properties, as shown in Table S4.

Conclusions
In summary, we have successfully designed defect-rich 3D ower-like MgCr-LDHs composed of 2D nanosheets by using a facile hydrothermal and light irradiation method, and taken advantage of these special 3D ower-like structures that provided added active sites, thereby behave as an effective photocatalysts by reducing the recombination of photo-induced e-and h+ pairs, for enhancing the water splitting activities. In addition, XPS and PL analyses shows the dominance of oxygen vacancies and defects site with special electronic con guration of Cr 3+ dopant (t 3 2g e 0 g ), and synergistically facilitates charge transfer, conductivity, electron capture and adsorption of water oxidation intermediates for facilitating the H 2 and O 2 production. Moreover, the MgCr-LDHs nano owers delivered interesting PEC properties with low Tafel slope values of 82 mV/decade for a current density of 6.9 mA/cm 2 , which is signi cant and these LDH might be used as a potent photoanode material for future PEC water splitting activities. Evidently, MgCr-LDH nano owers exhibited superior photocatalytic H 2 evolution activities of 1315 µmol/h, which was 1.8  In order to synthesized hierarchal ower like 3D MgCr-LDH, the recovered gel of 2D MgCr-LDH nanosheets was again redispersed and diffused in 10 mL of formamide solution and sonicated for 30 mins followed by stirring for 30 mins under N 2 atmosphere at room temperature. The formamide treated colloidal dispersion was transferred to 100 mL Te on lined autoclave reactor and treated at 80°C for 24 h. After cooling down to room temperature, the as-synthesized MgCr-LDH gel precipitate was slowly aged under the irradiation of visible light for 30 mins. Finally, the as-synthesized hierarchal 3D MgCr-LDH was washed with DI water and ethanol for three times and vacum dried overnight at 40°C for further use. The most signi cant feature of this proposed synthetic approach is the tremendous simpli cation and greener route of the synthesis procedure for the ower like 3D MgCr-LDH composed of 2D nanosheets.

Photocatalytic Water Splitting Measurement Studies
The catalytic competence of the as prepared MgCr-LDH samples were tested towards water splitting reaction under visible light exposure from 125 W Xe lamp (power density = 100 mW cm −2 ) attached to a quartz reactor tted with Julabo based chiller and 1 M NaNO 2 as UV cut off lter to lter out visible light of λ ≥ 400 nm. The water splitting reaction was begin with the addition of 0.02 g of catalyst to 20 mL of 10 vol% CH 3 OH solution and other sacri cial agents then purged with N 2 gas for 15 mins to remove any dissolved O 2 gas to make the environment inert prior to light exposure. Then the reaction suspension was stirred continuously for 1h to avoid any catalyst settlement under the exposure of visible light. The evolved gas was collected using downward displacement of water and further detected by GC-17A and column packed with 5 Å molecular sieves, set with thermal conductivity detector (TCD). Similar experiment condition was implemented, for O 2 evolution, with 0.03 g of catalyst added to 30 mL of 10 vol% of AgNO 3 and other tested sacri cial agents.

Apparent Conversion E ciency (ACE) for H 2 evolution =
The apparent conversion e ciency (ACE) of the MgCr-LDH/NF photocatalyst producing H 2 gas of 1315 µmol/h and O 2 gas of 579 µmol/h by using 125 W Hg lamp as the visible light source positioned 9 cm away from the photocatalytic reactor) could be determined by using the below mentioned equation (9)  Na on and sonicated for 20 mins. Then the mixture was coated onto uorine-doped tin oxide (FTO) by drop-casted method in the dimension of 1× 1 cm 2 . The LDH coated thin lms were vacuumed dried at 80°C for overnight prior to use.

PEC measurements
PEC measurements were carried out by potentiostat−galvanostat (IviumStat) terminal at a scan rate of 10 mV s −1 , with accessories of 300 W Xenon lamps of 100 mW/cm 2 illumination was maintained as the light source, three-electrode system carrying Pt, Ag/AgCl (3.5M KCl), and FTO, as counter, reference, and working electrode, respectively. 0.1 M Na 2 SO 4 solution with tested pH 6.5 was used as the electrolyte.

Materials characterization Techniques
The phase purity of the as-prepared materials were characterized by XRD, Rigaku Mini ex powder diffractometer) with Cu Kα as radiation source (λ = 1.54 Å, 30 kV, 50 mA). The functional groups associated with the bending and stretching mode of vibration of the materials were speci ed by JASCO FT-IR-4600, using KBr reference. The exterior surface morphology and structural features of the materials were obtained by FESEM by using ZEISS Sigma 500 VP microscope. The internal structure and morphology was explored under the TEM and HR-TEM by using JEOL 2100. The XPS measurement was taken at an X-ray photoelectron spectrometer (ESCALAB 250XI) with X-ray source as  Figure 1 Growth mechanism of MgCr-LDH nano ower.

Figure 4
Analysis of the XPS results of the deconvoluted XPS spectra of MgCr-LDH/NS and MgCr-LDH/NF for Mg2p, Mg2s, Cr2p, O2p, and C1s.        Relative band positions and charge transfer mechanism of the MgCr-LDH/NF for water splitting reaction under visible light exposure.