Fabrication of Zr-based Metal Organic Framework and Its Application in to Photocatalysis and Electrocatalysis by Dye Removal Studies

Fabrication of Zr-based Metal Organic Framework and Its Application in to Photocatalysis and Electrocatalysis by Dye Removal Studies

Mr. Harishkumar Sivaraju

Research Scholar, UG and Research Department of Chemistry, Erode Arts and Science College (Autonomous),

Erode 638 009 Tamil Nadu, India (Affiliated to Bharathiar University, Coimbatore-641046)

Dr. Santhi Mariappan

Associate Professor and Head, UG and Research Department of Chemistry, Erode arts and Science College (Autonomous),

Erode 638 009 Tamil Nadu, India

(Affiliated to Bharathiar University, Coimbatore – 641 046)

Abstract – In this work, the photocatalytic degradation of Acid Yellow 99 (AY 99) dye under solar light irradiation using the Zr- MOF@GNS and synthesized ZrO2 is investigated. The ZrO2 is synthesized using a Zr-metal precursor. The prepared ZrO2 is used to synthesize Zr-MOF@GNS, and it is further examined using FT-IR, PXRD, and FESEM analyses. The photocatalytic degradation of AY 99 is studied by varying the initial concentration of AY 99, the length of the irradiation period, the catalyst dose, and the pH of the dye solution. From the results, it is found that the Zr-MOF@GNS is an efficient and promising catalyst for the enhancement of photocatalytic degradation of AY 99. Electrochemical studies show that ZrO2 and Zr-MOF@GNS can act as electrocatalysts for acid-base titrations.

Keywords – ZrO2, Zr-MOF@GNS, AY 99, Photocatalysis, and Electrocatalysis.

Graphical abstract

INTRODUCTION:

The toxicity and environmental persistence of MB poses serious risks: One of the popular acidic dyes that is environmentally persistent, toxic, carcinogenic, and mutagenic is Acid Yellow 99 [[LINK:01]]. Health effects range from respiratory distress, tissue necrosis, and jaundice to oxidative damage, due to MBs monoamine oxidase inhibition and its water solubility. MB also enters food chains, impairing aquatic life and posing bioaccumulation risks [02]

Acid Yellow 99 (AY-99) is an azo dye commonly used in the textile and leather industries for its vibrant yellow color. The chemical structure of AY-99, which includes an azo bond (-N=N-) and aromatic rings, makes it highly resistant to biodegradation and other conventional removal methods. The persistence of such dyes in aquatic environments poses significant ecological and health risks. AY-99, like many azo dyes, is toxic to aquatic organisms and can disrupt ecosystems by decreasing oxygen levels in water bodies [3].

The coagulation process effectively decolorizes insoluble dyes, but it fails to work well with soluble dyes. Traditional methods for removing dyes from wastewater include adsorption, chemical oxidation, biological treatment, and membrane filtration [4]. However, these techniques often come with limitations such as incomplete dye removal, generation of secondary pollutants, or high operational costs. These methods are effective but tend to be energy-intensive and may generate toxic by-products [5].

Biological treatments, while environmentally friendly, may not be effective for recalcitrant dyes like AY-99 due to their complex molecular structure and resistance to microbial degradation. The long-term presence or accumulation of these dyes in wastewater discharged from these industries is detrimental to the aquatic environment. Given these challenges, the need for more efficient, sustainable, and eco-friendly methods to degrade organic pollutants, including dyes, is crucial [6].

Synthetic dyes are widely used in various industries such as textiles, leather, paper, plastics, and cosmetics, leading to the discharge of large volumes of dye-containing effluents into water bodies. These dyes, particularly methylene blue, crystal violet, and rhodamine B, as well as Acid Yellow 99, are not only toxic and carcinogenic but also resistant to biological degradation due to their complex aromatic molecular structures. Hence, effective and eco-friendly strategies for dye removal are essential for environmental protection.

Among several advanced oxidation processes, photocatalytic degradation has emerged as a highly promising method for dye removal due to its cost-effectiveness, reusability, and ability to completely mineralize dyes under light irradiation.

This process typically utilizes semiconductor photocatalysts (e.g., TiO, ZrO, g-CN) that, under UV or visible light, generate electron-hole pairs capable of producing reactive oxygen species (ROS) such as hydroxyl radicals (OH) and superoxide radicals (O), which attack and break down dye molecules into non-toxic end products like CO and HO.

Key Features and Efficiency of Photocatalytic Degradation

• Eco-friendly and non-selective: Can degrade a wide range of organic pollutants without the need for additional chemicals.
• Low energy requirement: Especially under visible light or sunlight, making it suitable for sustainable applications.
• No secondary pollution: Unlike adsorption or chemical treatments, photocatalysis does not produce harmful residues.
• Regenerability: Photocatalysts can be reused multiple times with minimal loss in activity.[7]

Recent studies have shown enhanced performance through doping, composite formation, and nanostructuring of photocatalysts. For instance, Ag-doped ZnO nanocomposites demonstrated superior visible-light-driven degradation efficiency for methylene blue dye, attributed to better charge separation and light absorption [8].

Photocatalytic degradation offers a viable solution by utilizing light energy to drive chemical reactions that break down harmful pollutants into non-toxic by-products, such as water and carbon dioxide [9]. However, the application of photocatalysis for environmental remediation is still a challenging task.

Metal-organic frameworks (MOFs) are a class of materials consisting of metal ions or clusters coordinated to organic ligands, forming a highly porous, crystalline structure. MOFs have gained significant attention as photocatalysts due to their high surface area, tunable structure, and ability to adsorb a wide variety of organic pollutants [10]. A metalorganic framework (MOF) possesses high surface area, chemical tunability, and abundant active adsorption sites, resulting in MOF adsorbents exhibiting high adsorption capacity for metal ions. These properties make MOFs ideal candidates for applications in environmental remediation, including photocatalytic degradation of dyes [11, 12]. Among various MOFs, zirconium-based MOFs (Zr-MOFs) have shown exceptional promise due to their high stability, large surface area, and ability to enhance photocatalytic performance.

Monoclinic ZrO, a key component in Zr-MOFs, is known for its excellent chemical

stability, thermal resistance, and low toxicity. These properties make ZrO-based MOFs highly suitable for photocatalytic applications, especially in harsh environments where other photocatalysts might degrade or lose activity. Zr-MOFs are also highly stable in aqueous environments, a critical factor for wastewater treatment processes [13-15].

Metal-Organic Frameworks (MOFs) have emerged as one of the most promising classes of photocatalysts for the degradation of organic dyes from wastewater. Due to their high surface area, tunable porosity, structural diversity, and photoactive metal centers, MOFs offer superior photocatalytic performance compared to conventional semiconducors like ZrO. MOFs such as MIL-125 (Ti), UiO-66 (Zr), and NH-MIL-88B (Fe) exhibit excellent light absorption and charge separation capabilities, facilitating the generation of reactive oxygen species (ROS) under UV and visible light [16].

Furthermore, the integration of light-harvesting linkers (e.g., amino-terephthalate) into the MOF structure enhances the absorption in the visible region, making them ideal for solar-driven dye degradation. Additionally, MOFs can be post- synthetically modified or composited with other materials like graphene oxide or noble metals to further enhance photocatalytic efficiency and stability [17]. Notably, studies have shown that MOF-based photocatalysts exhibit high degradation efficiency toward common dyes such as methylene blue, rhodamine B, and crystal violet, achieving nearly complete mineralization without generating toxic byproducts.

In addition, the crucial electronic and/or surface structures of hollow and/or porous nanostructures can be tuned with the use of

MOFs, thereby promoting electrocatalytic activity [18]. The present study focuses on photocatalytic applications of ZrO2 and Zr-MOF@GNS using AY99 dye as a model compound. The electrochemical studies were conducted using strong acid and strong base titrations.

1. EXPERIMENTAL

   Materials

   The AY99 dye is purchased commercially from local vendors (C25H19N4NaO8S2, CAS: 10343-58-5).

   De-ionized water is used in the process. Zirconium oxychloride is procured from Nice Chemicals (P) Ltd., and all other reagents are used as such without further purification.Synthesisof ZrO2

   ZrO2issynthesizedbymixing0.1MofNaOHwitha 2mMsolutionofmelamine and 2.5 mM of zirconium oxychloride in a magnetic stirrer. The loosely bonded metal residue is baked in a muffle furnace at 9000C for 12 hours to produce white powder after being repeatedly cleaned by centrifugation with alcohol.

   SynthesisofZr-MOF@GNS

   In order to accomplish carbonization, the sample is placed in a microwave oven and heated to around 500 °C for three hours while being exposed to oxygen. To oxidize the ZrO2, the sample is allowed to cool to ambient temperature before being annealed for an hour at 300 °C in an air oven.

   2.3Conductometricdetermination

   The conductometric titration of strong acid and strong base is carried out using ZrO2 and Zr-MOF@GNS as an electrocatalyst. 0.1 N of HCl is titrated with 0.5 N NaOH solution by adding 0.005 g/l of Zr-MOF@GNS. The values observed are plotted against when the volume of NaOH is added; from the curve obtained, the endpoint is determined graphically.

2. RESULTANDDISCUSSION

Characterization

The synthesized Zr-MOF@GNS and ZrO2 are examined using FT-IR, FESEM and PXRD analyses. The surface morphology of ZrO2 and Zr-MOF@GNS is characterizedusingascanningelectronmicroscope(SEM,HitachiS-4800).Fourier Transform Infrared Spectroscopy (FT-IR) measurements are carried out with a SHIMADZU IRTRACER 100, utilizing the KBr pellet method over a range of 4500500 cm1.The crystalline phase structure is confirmed viaPowder X-ray Diffraction(PXRD) using a BRUKER USA D8 Advance DaVinci diffractometer with Cu-K radiation, scanning from 0° to 100°. [19-20]

FT-IR Analysis

Fig1 FT-I Rspectral analysis of Zr-MOF@GNS and ZrO2 Fourier-Transform Infrared (FTIR)

Spectroscopy was employed to elucidate the chemical functional groups and confirm the successful synthesis of the Zr-

MOF@GNS nanocomposite. The spectra for the pristine ZrO and the Zr-MOF@GNS composite are presented in Fig. 01.

The synthesized ZrO nanoparticles exhibited a broad absorption band in the range of 400-700 cm¹, which is characteristic of the stretching vibrations of ZrO bonds [21]. Additional bands observed at 1630 cm¹ and 3400 cm¹ were assigned to the H OH bending and OH stretching modes of water molecules, respectively, a common feature in metal oxides with high surface area [22].

The incorporation of melamine was evidenced by the distinct NH stretching vibrations, manifesting as a series of medium- sharp bands in the 3200-3500 cm¹ region [23-24]. Furthermore, the C=C skeletal vibrations from the graphitic sp² carbon domains of the graphene nanosheets were identified as a shoulder near 1500-1600 cm¹, overlapping with the carboxylate signals. The ZrO stretching vibrations from the inorganic secondary building units (SBUs) of the MOF contributed to the broad absorption features below 800 cm¹ [25, 26].

The collective evidence from the FTIR analysis confirms the successful formation of the Zr-MOF structure, its functionalization with melamine, and its composite nature with graphene nanosheets [27].

PXRD Analysis

Fig3 PXR Danalysis of (a) ZrO2 and (b) Zr-MOF@GNS

Major diffraction peaks appear at 2 30.3°, 35.3°, 50.2°, 60.3°, and 62.8°, consistent with the tetragonal phase of ZrO (JCPDS 49-1642). The presence of sharp, well-defined peaks signals high crystallinity and phase purity, which directly correlates with the photocatalytic activity of ZrO-based systems. Such nanoscale, highly crystalline ZrO is acknowledged for efficient charge separation during photocatalytic reactions, a factor repeatedly demonstrated as enhancing pollutant degradation rates.

Particle size calculation using the Scherrer equation

D = 0.9 / Cos

yields an average size around ZrO is 0.1101 nm and Zr-MOF@GNS is 0.1022 nm, a result that closely matches values commonly reported for ZrO nanostructures and GNS product formed. The subtle peak near 2 26.5° is attributed to the (002) plane of graphitic carbon, evidencing successful incorporation of graphene nanosheets within the composite. The absence or attenuation of some parent ZrO peaks and the emergence of new or broadened reflections in the composite confirm the formation of the zirconium MOF structure and integration with GNS. Zr-MOF composites regularly highlight MOF-related manifold reflections spanning 2 2030°, further supporting successful framework formation. [28-35]

FESEM Analysis

2a 2b

Fig 2a and 2b ZrO SEM (2a), Zr-MOF SEM (2b) Images

The Fig. 2a depicts that the ZrO particles are in an amorphous state. Due to aggregating or overlapping of smaller particles, there are some larger particles. The SEM pictures clearly exhibit that the grains are randomly distributed with smaller size, and it is noticed that the particles are of homogeneous spherical shape, and are dusty and messy with sharp edges in the corners and are slightly crystalline. The above images provide information on the shape and structure of the particles synthesized [[LINK:36]]. Fig. 2b evidences that the synthesized Zr-MOF@GNS is in a crystalline state and is brittle in nature. The improved morphology is advantageous for photocatalytic and electrocatalytic applications as it ensures a higher surface area and better accessibility to active sites.

Photocatalytic Application

Effect of Initial Concentration of AY99 Dye

In order to study the effect of initial concentration of solution by the catalysts such as ZrO and Zr-MOF@GNS, the amount of the catalysts dosage is kept constant and different initial concentrations are varied in the particular time interval.

Fig 4 – Effect of Initial Concentration on the Photodegradation of AY99 Under Solar Light Irradiation

The observed results revealed that the quantum yield of the photocatalysis process increases with increase in initial concentration ofAY99 dye.This can be explained in terms of availability of active sites on the catalyst surface and the

penetration of solar light into the suspension. The total active surface increases in the AY99 increases with increase in the concentration which tends to increase in the removal [37].

The length of the irradiation period

In order to study the length of irradiation period on the removal of AY99 dye by photocatalytic degradation process on ZrO2 and Zr-MOFGNS, the degradation experiments are carried out at constant amount of the catalysts and optimum initial concentration but varying the irradiation period.

Fig 5: The Length of the Irradiation Period of the Photodegradation of AY99 Under Solar Light Irradiation

The length of the irradiation period plays an important role in the degradation process of pollutants. The length of the irradiation period varies from 30 to 180 min. The results reveal that the quantum yield of the reaction decreases with an increase in the irradiation period. It is observed that the Zr-MOF@GNS / solar light system exhibits better photocatalytic efficiency than that of the ZrO2 / solar light system. [38]

The catalyst dose

To study the effect of the catalyst dose, the amount of ZrO2 and Zr-MOF@GNS is varied, the initial concentration ofAY99 dye solution in each case is kept constant at optimum value.

Fig6 – The Catalyst Dose Variation of the Photodegradation of AY99 Under Solar Light Irradiation

Optimizing the amount of ZrO2 and Zr-MOF@GNS is done in order to obtain the maximum amount of quantum yield in the photocatalytic degradation process. Hence, in this study, the quantity of the catalyst is varied from 0.005 g/L to 0.025 g/L. It is noticed that the quantum yield of the reaction increases with an increase in the amount of ZrO2 and Zr-MOF@GNS. This is due to the fact that an increase in the catalyst dose increases the number of active sites on the surface of the catalyst, and hence the quantum yield of the reaction increases. [39]

The pH of the dye solution:

The photo degradation experiments are carried out at different initial pH at constant optimum initial concentration of AY99 dye solution, the amount of the catalyst and irradiation time.

Fig7 – The photodegradation of AY99 is varied on the photodegradation of AY99 under solar light irradiation.

The pH value is one of the important factors influencing the quantum yield of the photodegradation process of pollutants, especially dyes. The AY99 dye degradation is highly pH dependent. The photocatalytic degradation of AY99 dye at different pH values varying from 1 to 5 clearly shows that the quantum yield efficiency is higher in acidic medium. The Zr-MOF@GNS having high efficiency for photodegradation of AY99 dye. [40]

4.0 Pictorial Explanation

Fig9

4.1 Mechanism of Photodegradation of AY99 Dye

Using a photocatalystusually a semiconductor material like TiO2, ZnO, or ZrO2to capture solar energy and break down

dye molecules is known as photocatalytic degradation of hazardous dyes under sunlight. The photocatalyst creates electron- hole pairs by absorbing photons when exposed to sunshine. Reactive oxygen species (ROS) like hydroxyl radicals (OH) and superoxide anions (O2) are created when excited electrons (e) and holes (h) engage in redox processes. When the dye molecules are attacked by these very reactive species, they are broken down into smaller, less hazardous, or mineralized chemicals like CO2 and H2O. This environmentally friendly technique efficiently eliminates colors from wastewater.

Photons from sunlight with energy equal to or higher than its bandgap are absorbed by zirconium oxide. Positively charged holes (h) were left in the valence band after these energized electrons (e) in the valence band were promoted to the conduction band.

The electron-hole pairs interact with molecules in their environment. The holes (h) can oxidize water (H2O) or hydroxide ions (OH) to generate highly reactive hydroxyl radicals (OH). Simultaneously, the electrons (e) reduce oxygen molecules (O2) to form superoxide anions (O2).

The dye molecules are attacked by the very reactive superoxide anions (O -) and hydroxyl radicals (OH). By cleaving bonds

and starting oxidation processes, these radicals split the complex colour molecules into smaller pieces.

AY99+OH/O -CO +H O+otherharmlesscompounds (5)

The dye eventually mineralizes into non-toxic substances including carbon dioxide (CO2), water (H2O), and inorganic ions as a result of the intermediates created during the breakdown process being further oxidized over time. The photocatalyst is not consumed during the reaction, making the process sustainable. It can continue to catalyze reactions as long as it is exposed to sunlight and is not fouled or degraded.

This process is eco-friendly and efficient for treating dye-contaminated wastewater, as it utilizes sunlight (a renewable energy source) and does not produce harmful secondary pollutants. [[LINK:index:url:text]]

5.0 Application to Conductivity

From the catalysts which are used in the photocatalytic degradation, they are verified in order to check the conductivity of the synthesized photocatalyst.

Fig8 Electro chemicals tudies of ZrO2andZr-MOF@GNS

6. CONCLUSION:

The photocatalytic activity of ZrO2 and Zr-based MOF is studied under sunlight as a source for the photodegradation of AY99 dye. The effect of initial concentration, the length of irradiation time, catalyst dose used, and pH of the dye solution is studied and the reaction conditions are optimized.

The mechanism of photocatalytic degradation takes place through the formation of reactive oxygen species (ROS) such as hydroxyl radicals (OH) and superoxide anions (O2). The electrochemical studies indicate that Zr-MOF@GNS increase the rate of the reaction.

From the results, it can be concluded that Zr-MOF@GNS catalyst and ZrO2 act as photo and electro catalysts.

ACKNOWLEDGEMENT

The authors thanks, The Management and The Principal of Erode Arts and Science College (Autonomous), Erode,

638009 Tamil Nadu, India for providing the necessary facilities to complete this work.

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