Synthesis and characterization of lead-based metal–organic framework nano-needles for effective water splitting application

Metal organic frameworks (MOFs) are a class of porous materials characterized by robust linkages between organic ligands and metal ions. Metal–organic frameworks (MOFs) exhibit significant characteristics such as high porosity, extensive surface area, and exceptional chemical stability, provided the constituent components are meticulously selected. A metal–organic framework (MOF) containing lead and ligands derived from 4-aminobenzoic acid and 2-carboxybenzaldehyde has been synthesized using the sonochemical methodology. The crystals produced were subjected to various analytical techniques such as Fourier-transform infrared spectroscopy (FT-IR), Powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), Brunauer–Emmett–Teller (BET), and thermal analysis. The BET analysis yielded results indicating a surface area was found to be 1304.27 m2 g−1. The total pore volume was estimated as 2.13 cm3 g−1 with an average pore size of 4.61 nm., rendering them highly advantageous for a diverse range of practical applications. The activity of the modified Pb-MOF electrode was employed toward water-splitting applications. The electrode reached the current density of 50 mA cm−2 at an overpotential of − 0.6 V (vs. RHE) for hydrogen evolution, and 50 mA cm−2 at an overpotential of 1.7 V (vs. RHE) for oxygen evolution.


Scientific Reports
| (2023) 13:12531 | https://doi.org/10.1038/s41598-023-39697-z www.nature.com/scientificreports/ are recognized as electrochemically active catalysts that are extensively employed in electrochemical applications such as fuel cells, Li-batteries [26][27][28] , supercapacitors [29][30][31] , and water splitting 32,33 . This is in accordance with previous studies [34][35][36] . The utilization of MOF as a substrate for urea electrooxidation has been reported to be effective in urea removal. This is attributed to the substrate's extensive surface area, abundance of adsorption sites, proficient charge transfer capacity, and notable crystallinity, as documented in previous studies [37][38][39][40] . The oxygen evolution reaction (OER) is a crucial process in various devices such as rechargeable metal-air batteries, water electrolysis systems, and solar fuel devices, as it acts as the limiting factor due to its excessively high overpotentials.
Electrocatalysis for the oxygen evolution reaction (OER) holds significant importance within the realm of advanced technologies, as it serves as a pivotal factor in enhancing the efficiency of gas evolution. Consequently, a multitude of novel electrocatalysts have been devised to further augment this process. Considerable resources have been dedicated to the pursuit of efficient electrocatalysts, prompting the development of novel methodologies for studying material properties and the underlying mechanisms of the oxygen evolution reaction (OER) 41 . Recently, knowledge not only serves as the basis for understanding the functioning of the OER mechanism, but also highlights the essential factors that contribute to the efficacy of an electrocatalyst, as evidenced by numerous research investigations 42,43 .
The optimization of the hydrogen evolution reaction (HER) and oxygen evolution (OER) is deemed crucial for the generation of hydrogen, the process of water splitting, and the functioning of metal-air batteries. Over the past ten years, researchers have explored the potential of non-precious electrocatalysts that utilize transition metals such as nickel, cobalt, and copper, as well as their respective oxides, for the purpose of water-splitting. This has been documented in various studies [41][42][43][44] . One of the possible candidates is lead (Pb), which is a widely available and low-cost metal with high electrical conductivity and chemical stability 44 . Pb has been used as a catalyst for various electrochemical reactions, such as carbon dioxide reduction, oxygen evolution, and organic oxidation 45,46 . However, its application for EHE has been relatively less explored. Pb-MOF for electrocatalytic hydrogen evolution is a novel material that has attracted attention for its potential application in clean energy production. Pb-MOF is a metal-organic framework composed of lead (Pb) metal centers and organic ligands that form a porous crystalline structure. Pb-MOF can act as an efficient electrocatalyst for the hydrogen evolution reaction (HER), which is the process of splitting water into hydrogen and oxygen using electricity. Pb-MOF has several advantages over conventional HER catalysts, such as high surface area, high activity, low cost, and tunable properties. Pb-MOF can also be modified by doping with other metals or heteroatoms to enhance its conductivity, stability, and catalytic performance. By incorporating different ligands and metal components into the MOF structure, the catalytic activity and stability of lead-based MOFs can be enhanced and tailored for different HER conditions. Pb-MOF for electrocatalytic hydrogen evolution is a promising material that could pave the way for the development of sustainable hydrogen economy. Herein, we prepared a novel Pb-MOF composite using the ultrasonic assisted method. Then the modified composite is used for water splitting application by electrochemical approaches.

Experimental
Synthesis of Schiff base ligand (H 2 L) linker. By combining a saturated ethanolic solution of phthalaldehydic acid (5 g) with another saturated ethanolic solution, the previously prepared Schiff base ligand (H2L) was created (see Fig. 1).
By a ratio of 1:1, of 4-aminobenzoic acid (5.47 g). The amine was mixed with the aldehydic solution before being allowed to reflux for 5-7 h. The resulting ligand was filtered, and the filtrate was repeatedly washed with cold ethanol until it was transparent. The solid ligand was dried over anhydrous calcium chloride in a desiccator. Figure 1 illustrates that the yield rate was 87 percent. www.nature.com/scientificreports/ Ultrasonic synthesis of Pb-MOF. The Schiff base ligand, denoted as H2L and having a molar quantity of 1 g and 3.7 mmol, was solubilized in 50 mL of absolute ethanol. A solution of lead acetate dihydrate (0.7 g, 1.85 mmol) was prepared by dissolving it in 30 mL of ethanol. The molar ratio of Lead acetate dihydrate to H2L is 1:2. The amalgamation of the two solutions was conducted, followed by their placement in a receptacle that was submerged in a water bath. The mixture was then subjected to sonication for a duration of 60-75 min, with a frequency of 40 kHz and alternating 1-s intervals of activation and deactivation. The experiment maintained a constant ultrasonic output of 60 watts. Following the required reaction time, the product was subjected to ultrasonic irradiation, after which it was isolated via centrifugation. The resulting precipitate underwent a thorough washing process using 50 mL of water and 10 mL of ethanol, which was repeated thrice. Finally, the precipitate dried at a temperature of 130 °C for a duration of 12 h. Subsequently, the product was cooled under ambient conditions at room temperature. Schematic representation of the preparation of Pb-MOF illustrated in Fig. 2.

Results and discussion
Characterization of Schiff base ligand (H 2 L). The synthesis of a pre-existing ligand was conducted for the purpose of serving as an organic linker in the formation of metal-organic frameworks (MOFs) 47 . The ligand, denoted as H2L, was synthesized through the process of condensation between 4-aminobenzoic acid and 2-carboxybenzaldehyde. The facile methodology employed yielded a white-colored ligand, namely 2-(((4-carboxyphenyl)imino)methyl) benzoic acid. The elemental analysis results for carbon, hydrogen, and nitrogen were obtained, revealing percentages of 67.10% (calculated value = 66.91%), 5.36% (calculated value = 5.20%), and 4.20% (calculated value = 4.12%), respectively. The outcomes were in concurrence with the computations derived from the prescribed equation (C 15 H 11 NO 4 ), and the Schiff base ligand that was produced exhibited a distinct melting point of 270 °C, thereby validating its purity. As shown in Fig. S1, The IR spectrum analysis of the unbound ligand (H 2 L) revealed the absence of NH 2 bands of 4-aminobenzoic acid and the emergence of a fresh v(CH=N) azomethine band at 1601 cm −1 , as reported in reference 48 . The stretching bands of ν asym (COO-) and ν sym (COO-) were observed at 1468 cm −1 and 1321 cm −1 , respectively, as reported in reference 21 . As shown in Fig. S2, The ligand's 1H-NMR spectrum exhibited a singlet signal at 5.8 ppm for HC=N with a single hydrogen atom, a singlet signal at 12.3 ppm for carboxylic protons with two hydrogen atoms, and multiple signals in the 6.5-7.9 ppm range that corresponded to the ligand's aromatic protons. The mass spectrum of the Schiff base ligand under investigation was primarily characterized by molecular ion peaks of moderate to somewhat high intensity. As per the findings of elemental investigations, it was observed that the mass spectrum of the Schiff base ligand exhibited a distinct parent peak at m/z = 269.07 amu, which was in agreement with the ligand moiety C 15   in the 3435-2918 cm −1 range is attributed to the O-H vibrations of water molecules that are present in the crystal structure, as reported in reference 4 . As shown in Fig. 3, The Fourier Transform Infrared (FTIR) spectrum of the Pb-MOF exhibited the emergence of robust bands at 1394 and 1602 cm −1 , which can be attributed to the symmetric and asymmetric stretching modes of the coordinated (-COO) group, respectively. The observation suggests that the carboxyl group (-COOH) of H 2 L is involved in the coordination with lead. The carboxylate group exhibits bidentate chelating coordination in the Pb-MOF due to the fact that its Δv(COO) (Δν = νas(COO) − νs(COO)) is greater than that of the H2L ligand. The observed difference in antisymmetric and symmetric carbonyl stretching frequencies Δν for Pb-MOF was 247 (Δν = 1602 − 1394 = 208), which is significantly greater than that of H2L (Δν = 1468-1321 = 147). This indicates that the carboxylate moieties in the lead MOF coordinate in a chelating bidentate mode. A novel peak denoting the Pb-O bond was detected at 539 cm −1 in the metal organic framework under investigation 2 .
Powder X-ray diffraction pattern (PXRD). The X-ray diffraction (XRD) technique is highly valuable in furnishing data pertaining to the structure, average grain size, crystallinity, and other structural parameters. The X-ray powder diffraction pattern of the synthesized Pb-MOF exhibited a high degree of structural crystallinity, as depicted in the accompanying figure.  (Fig. 4) The X-ray diffraction (XRD) pattern of the Pb-based metal-organic framework (MOF) obtained in this study is consistent with the findings reported in prior research 49,50 . Additionally, chemical structure of the Pb-MOF after stability test for 5 h of gas production. Thus, chemical structure changed that the PbO observed to generate instead of Pb-MOF. Severn characteristic peaks observed at 2θ equal to 18°, 29°, 31°, 36°, 49°, 54°. 60° according to reference card JCPDS card 01-078-1665 51,52 (see Fig. S3). However, MOF structure changed to corresponding oxides with higher oxidation states. Whereas, various oxidation states of lead enhance the ability of the materials toward water splitting application.  www.nature.com/scientificreports/ the conversion of the hybrid materials or organometallic materials through the catalysis process were extensively studied in literature to find out explanations for structure stability 53 . The chemical structure of the prepared Pb-MOF was finally estimated as represented in Fig. S4.
BET. The synthetic Pb-MOF's surface area and porosity were measured volumetrically using N 2 adsorption. Standard N 2 adsorption-desorption tests were performed at 77 K in order to look into the surface area, pore volume, and pore structure of Pb-MOF, as shown in Fig. 5. A type IV isotherm, which is typical of mesoporous materials, was seen in Pb-MOF. It was determined that the BET surface area was 1304.27 m 2 g 1 . With an average pore size of 4.61 nm, the total volume of pores was calculated to be 2.13 cm 3 g 1 .
SEM image of Pb-MOF. The appearance, size, and structure of the sonochemically produced Pb-MOF were examined using scanning electron microscopy (SEM). It demonstrates that the particle size produced by ultrasonic irradiation is less than that produced by the solvothermal synthesis approach 3 . SEM images of the created Pb-MOF are shown in Fig. 6a. The results demonstrated that rod-shaped nanoparticles in the 53-83 nm range were successfully produced. Furthermore, the confirmation of Pb-MOF preparation was confirmed by comparing with unmodified ligand (see Fig. 6b). www.nature.com/scientificreports/ electrode was activated in the solution to generate the electrochemical active species. Thus, activation step was performed in alkaline medium in potential range of 0 to 1.5 V (vs. RHE). As represented in Fig. 9, repeated 50 CVs of modified GC/Pb-MOF electrode at scan rate of 50 mV s −1 in solution of 1.0 M NaOH. Two redox peaks were observed; 1st oxidation peak at potential of 0.9 V attributed to conversion of Pb 2+ to Pb 3+ while the 2nd oxidation peak at potential of 1.1 V attributed to conversion of Pb 3+ to Pb 4+ . Additionally, the reduction peak    The process of oxygen evolution is of paramount importance in the conversion of chemical energy to electrical energy in fuel cells and batteries 54 . Various electrochemical methods have been utilized to determine the mechanism of the oxygen evolution reaction. One of the prevalent pathways for the electrochemical conversion of hydroxide to molecular oxygen involves a two-step electrochemical process. The initial step involves the adsorption of hydroxide ions onto the electrode surface, leading to the formation of OH ads species. Subsequently, the adsorbed hydroxide group interacts with hydroxide ions present in the surrounding medium, resulting in the production of O ads . The ultimate stage involves the release of adsorbed atomic oxygen, resulting in the production of molecular oxygen. The operational framework of oxygen evolution reaction (OER) was established in the follows 42 : Figure 10a illustrates the Oxygen evolution reactions (OER) observed on the GC/Lead Metal-Organic Framework (GC/Pb-MOF) under the influence of a 1.0 Molar concentration of Sodium Hydroxide (NaOH). As per the findings, a singular oxidation peak can be attributed to Pb 2+ and Pb 4+ ions, occurring at a potential of 0.5 and 1.05 V (vs. RHE) respectively 55,56 . The observed current density for OER in the Pb-MOF sample was found to be high. The modified electrodes exhibited a notable increase in current density, with the current peak attaining   Figure 10b depicts a Tafel plot of the GC/Pb-MOF electrode for oxygen evolution reaction. The Tafel slopes pertaining to distinct modified surfaces have been determined, with a value of 64.4 mV dec −1 being obtained for GC/Pb-MOF. The Tafel slope for GC/ Pb-MOF found to be comparable with other modified surfaces for OER in alkaline medium like NiCo nanosheets (41 mV dec −1 ) 57 , GC/LiCoO 2 (48 mV dec −1 ) 58 , and GC/NiFe 2 O 4 (98 mV dec −1 ) 59 respectively. The study focused on the investigation of hydrogen evolution reactions on a surface that has been modified by GC/Pb-MOF. The modified electrode's linear sweep voltammetry in a solution of 1.0 M NaOH is depicted in Fig. 11a.
The Pb-MOF that underwent modification exhibited a notable increase in current density, which can be attributed to the incorporation of organic molecules into the electrocatalyst frameworks. This modification resulted in an enhancement of both electronic and adsorption properties.
The subsequent mathematical expression has the capability to yield the hydrogen evolution reaction (HER) in an extremely basic environment 60,61 : The initial stage of the hydrogen evolution reaction (HER) entails the adsorption of hydrogen ions (also known as the Volmer step) on the electrode's surface. Subsequently, the subsequent stage involves the amalgamation of two hydrogen ions that are adsorbed on the surface, which is commonly referred to as the Tafel step. Alternatively, it may involve the direct bonding between a hydrated proton present in the medium and an adsorbed hydrogen atom on the surface, which is known as the Heyrovsky step.
The determination of whether the first or second step is the rate-determining step for hydrogen evolution reactions can be approximated through the utilization of the Tafel polarization curve in the context of linear sweep voltammetry. Figure 11b depicts a Tafel plot of the GC/Pb-MOF electrode for hydrogen evolution reactions. The Tafel slopes pertaining to distinct modified surfaces have been determined, with a value of 76 mV dec −1 being obtained for GC/Pb-MOF. The provided Tafel slope value for GC/Pb-MOF matched with other reported for modified surfaces like Ni 2 Fe/N-doped porous C(83 mV dec −1 ) 62 , NiFe-LDH/MXene/Ni foam(70 mVdec −1 ) 63 , and Ni/NiO core/shell nanosheets(43 mV dec −1 ) 64 .
Else, stability of the modified electrode toward gas production (i.e., OER, and HER) was investigated in alkaline medium using constant potential chronoamperometry. As represented in Fig. 12a, the durability of surface for oxygen evolution in 1.0 M KOH were tested for 5 h at potential of 1.7 V (RHE). The current decreased by 6.5% after 5 h. However, the stability of the modified electrode toward hydrogen production process was performed at potential of − 0.6 V (vs. RHE). Thus, the current of the electrode decreased by 13.8% of the initial values (see Fig. 12b).
The utilization of electrochemical impedance spectroscopy (EIS) was implemented in order to investigate the hydrogen and oxygen evolution phenomena in relation to the modified GC/Pb-MOF electrodes. The Nyquist plot for the GC/Pb-MOF modified electrodes in a 1.0 M NaOH solution at an AC potential of 1.6 V (vs. RHE) is depicted in Fig. 13a. The sample of MOF-Lead exhibited a semi-circular response in relation to the oxygen evolution reaction. The Nyquist plot's semi-circle is indicative of the charge transfer process. The EIS data pertaining to the process of oxygen evolution was subjected to fitting procedures utilizing NOVA software. The modified electrode's fitting circuit is represented by two resistance components that pertain to the solution resistance Step-water dissociation   Fig. 13b, where a constant AC potential of − 0.6 V vs. Ag/AgCl was employed. The Nyquist plot obtained from the modified electrode GC/ Pb-MOF exhibited similar semi-circular plots, albeit with varying resistance magnitudes. The electrochemical production process may be regarded as a purely charge transfer phenomenon, as exemplified in the electrochemical impedance spectroscopy (EIS) data. The EIS outcome was subjected to fitting utilizing NOVA software. The GC/Pb-MOF electrode manifested a dual circuit configuration comprising a single cell linked to the solution resistance. The said cell consisted of a parallel connection between a resistance and the CPE (see Fig. 13 inset). Furthermore, the presence of a constant phase element suggests the existence of surface heterogeneity, which has an impact on the efficiency of hydrogen generation. The charge transfer resistance of the GC/Pb-MOF electrode is 570 Ω. High electrode activity is associated with a lower resistance value. The fitted parameters of EIS result were reported in Table. 1.