GRADUATION THESIS MAJOR: MATERIALS TECHNOLOGYINSTRUCTOR: PHAM THANH TRUC Ho Chi Minh city, August 2024 STUDYING EFFECT OF POLYSTYRENE NANOSPHERES ON PHOTOCATALYTIC PROPERTIES OF BISMUTH
INTRODUCTION
Overview
In recent years, the establishment of new industrial parks in Vietnam has significantly boosted the country's economy; however, this rapid industrialization has led to severe environmental issues, including water, soil, and air pollution Manufacturing waste, particularly from the textile, dyeing, and paper industries, is a major contributor to water pollution, posing serious risks to both human and environmental health due to the presence of organic compounds and heavy metals in these pollutants.
The textile dyeing industry, a longstanding sector vital to economies like Vietnam's, faces significant environmental challenges due to its reliance on toxic synthetic dyes and hazardous production processes A staggering 15% to 20% of wastewater generated during dyeing is often discharged untreated into water bodies, leading to severe ecological and health repercussions Commonly used toxic substances include sulfur, heavy metals, and formaldehyde, contributing to the toxicity of dyeing wastewater Annually, approximately 800,000 tons of dyes are produced globally, with 10% to 15% wasted during manufacturing Most dyes today are synthetic and non-biodegradable, posing cancer risks to humans Addressing the pollution caused by dyeing remains a pressing concern for researchers and scientists worldwide.
Public concerns regarding water treatment methods have increased, particularly in the challenge of eliminating dye contaminants Biological treatments encounter significant obstacles due to the toxic effects of dyes on bacterial growth Additionally, flocculation methods prove to be ineffective, especially at high dye concentrations.
Thus, the identification of an effective approach towards eradication of dye color and its toxicity from wastewater is a significant challenge[4].
Studies on photocatalytic materials
1.2.1 The demand of photocatalysts around the world
In recent years, the use of TiO2 as a photocatalyst in water treatment has gained significant attention due to its ability to degrade harmful dyes such as Methyl Red, Methylene Blue, and Congo Red when suspended in water and exposed to UV radiation This process not only bleaches these dyes but also transforms toxic substances into non-toxic materials, effectively purifying the water Research indicates that doping TiO2 with nonmetal ions like boron, carbon, nitrogen, and fluorine enhances its catalytic activity, allowing it to utilize visible light—a more abundant solar energy source—thereby improving the efficiency of photocatalytic reactions These advancements are anticipated to contribute to the development of more efficient and sustainable photocatalytic systems.
Recent studies have explored various strategies to enhance the photocatalytic activity of TiO2, including acid treatment to modify its phase, crystallinity, and surface area, ultimately increasing the number of catalytic sites In addition to TiO2, zinc oxide (ZnO) has emerged as a promising photocatalyst, particularly when synthesized via the sol-gel method at varying temperatures, demonstrating exceptional efficiency in degrading Azo dyes for long-term water treatment applications The photocatalytic activity of doped ZnO can be fine-tuned by altering its band gap through the incorporation of different elements, affecting its crystal structure, electronic properties, and surface characteristics Furthermore, the creation of ZnO hollow nanospheres using polystyrene microspheres as templates has led to materials that effectively absorb ultraviolet light while minimizing visible light absorption, enhancing their light interaction and scattering properties.
In addition to ZnO, other semiconductor materials like TiO2, BiOCl, and BiOBr have garnered significant attention for their photocatalytic properties, with their structural and electronic characteristics playing a crucial role in their effectiveness Extensive research has focused on understanding the mechanisms behind these materials and exploring their applications in environmental and energy challenges Among the bismuth halide oxides (BiOX, where X = Cl, Br, I), BiOBr stands out as a highly efficient photocatalyst for processes such as contaminant removal, hydrogen generation, and nitrogen reduction Its internal electric field facilitates effective charge carrier separation, enhancing its photocatalytic capabilities To further improve BiOBr's performance in practical applications, advancements such as elemental doping and the development of two-dimensional materials are essential.
1.2.2 The demand of photocatalysts in Vietnam
Ms Nguyen Hong Hanh from the Hanoi University of Science and Technology has successfully synthesized porous coral-like ZnO nanoplates using a hydrothermal method These optimized nanostructures exhibit exceptional photocatalytic activity for degrading RhB dye under sunlight The high efficiency is attributed to the unique characteristics of the nanoplates, including their structure, crystallinity, and surface defects This research highlights the potential of ZnO nanoplates for applications in sustainable water treatment technologies.
Tran Hoang Tu from the Ho Chi Minh City University of Natural Sciences has developed a composite material consisting of Fe2O3, TiO2, and graphene oxide (GO) using hydrothermal processing This technique produced nanoparticles with sizes in the tens of nanometers range Notably, the synthesized Fe2O3-TiO2/GA demonstrated impressive efficacy in the photodegradation of methylene blue (MB) dye, achieving a degradation rate of 97% under UV light.
Luong Hoai Nhan from Ho Chi Minh City University of Science has successfully deposited Mn-doped ZnO nanorods on glass substrates The incorporation of Mn ions into the ZnO crystal lattice reduces the band gap, extending it into the visible light spectrum and enhancing the generation of electrons and holes, which improves photocatalytic activity.
Motivation and Objectives
This study aims to explore the interaction of polystyrene nanospheres with BiOBr photocatalysis, a promising technology for degrading organic compounds in environmental applications The photocatalytic efficiency of BiOBr is often limited by its surface characteristics and light absorption capabilities By incorporating polystyrene nanospheres, we can enhance the structural and optical properties of BiOBr, potentially improving its photocatalytic activity This advancement may lead to innovative methods for optimizing these materials, making them more effective in converting sunlight into usable energy.
The primary objectives of this thesis are as follows:
Suggesting a synthesis method for BiOBr materials involving NPS as a hard template which permits the creation of an internal cavity in BiOBr material
This study focuses on synthesizing BiOBr using a specific mass percentage of NPS, aiming to analyze its structural properties, surface area, and adsorption capacity Additionally, it seeks to enhance the photocatalytic activity of BiOBr under sunlight, comparing its performance with BiOBr produced through alternative methods.
Utilizing these materials in wastewater treatment significantly lowers costs and time, contributing to the preservation of water environments in Vietnam and globally, thereby preventing further degradation.
The structure of the thesis includes:
Chapter 1: Overview of the topic
This study focuses on the synthesis of hollow BiOBr using NPS materials, highlighting the significance of NPS in the process It investigates how different mass percentages of NPS influence the properties and applications of BiOBr, aiming to enhance its performance and functionality.
The characterization of NPS, BiOBr, and hollow BiOBr structures was conducted using various advanced techniques, including Field Emission Scanning Electron Microscopy (FE-SEM), Dynamic Light Scattering (DLS), Fourier Transform Infrared Spectroscopy (FT-IR), X-ray Diffraction (XRD), and Ultraviolet-Visible Spectroscopy (UV-Vis).
Investigate the photocatalytic performances of hollow BiOBr materials as well as that of pure BiOBr for the photodegradation of Rhodamine B under visible light for
THEORETICAL BASIS
Overview of photocatalysis
Photocatalysis is a light-activated process that accelerates chemical reactions using substances known as photocatalysts, primarily made of semiconductor materials When exposed to light, these photocatalysts generate electron-hole pairs within their lattice structure, which then trigger redox reactions on the catalyst's surface The effectiveness of this catalytic activity relies on the efficient separation and transfer of these photoinduced charge carriers, along with the photocatalyst's surface chemistry.
The photochemical process, as depicted in Figure 2.1, initiates when photons with energy surpassing the photocatalyst's band gap are absorbed, resulting in the formation of an electron-hole (e–/h+) pair If the generated potential exceeds 2.31 V/NHE at pH=0, oxidative holes (h+) oxidize water, yielding hydrogen ions (H+) and hydroxyl radicals (OH•) Conversely, reductive electrons (e-) can reduce oxygen (O2) to superoxide radicals (O2•-) when the potential is below 0.92 V/NHE at pH=0 Hydroxyl radicals, with an oxidation potential of 2.31 V/NHE, can cleave carbon-carbon bonds in organic molecules, while superoxide radicals can indirectly produce additional hydroxyl radicals through hydrogen peroxide (H2O2) decomposition Although both radicals aid in degradation, the direct production of hydroxyl radicals via water oxidation is considered the more effective pathway.
The overall photocatalytic degradation of organic compounds can be summarized by the following equation:
OH* + Organic pollutants → CO2 + H2O + Other products
Semiconductors serve as effective catalysts in photochemical reactions, enhancing the decomposition of organic compounds while remaining unchanged themselves This process is significant as it produces little to no harmful byproducts, ensuring a safer environmental impact.
Introduction of BiOBr
Bismuth, a naturally occurring element known since the 17th century and often referred to as the "green element" due to its low toxicity, has numerous industrial applications Recently, bismuth compounds, particularly bismuth oxybromide (BiOBr), have gained significant attention for their unique properties and potential applications BiOBr, a semiconductor from the ternary semiconductor group V-VI-VII, features an indirect bandgap of 2.61–2.90 eV, making it suitable for photocatalytic applications Its distinct electronic properties and layered structure enhance efficiency in visible-light absorption and electron-hole pair generation BiOBr belongs to the BiOX compounds (where X = F, Cl, Br, and I), all of which exhibit a tetragonal layered crystal structure, consisting of alternating [Bi2O2] and [X] layers, with strong bonding between bismuth and oxygen atoms in each layer.
The structural arrangement of bismuth-oxygen and halogen layers creates a non-homogeneous charge distribution, which is held together by van der Waals forces This arrangement generates an internal electric field that is crucial for enhancing photocatalytic activity, as it promotes the separation of hole-electron pairs during the process.
Bismuth oxybromide (BiOBr) has emerged as a promising material for photocatalytic applications due to its unique layered structure, excellent chemical stability, and narrow bandgap that allows for efficient visible light absorption Its indirect bandgap structure minimizes electron-hole pair recombination, enhancing photocatalytic performance The tetragonal layered configuration of BiOBr, similar to PbFCl, consists of [Br-Bi-O-Bi-Br] layers interleaved with [Bi2O2] and Br double layers, contributing to its stability Recent studies indicate that the (001) crystal facet of BiOX materials (where X = Cl, Br, or I) is the most active site for photocatalytic reactions, as its exposure improves the separation and transfer of photogenerated charge carriers Additionally, the weak van der Waals forces holding the layered structure together facilitate easy exfoliation into two-dimensional sheets, further enhancing its photocatalytic capabilities.
Figure 2.2 BiOBr unit cell structure
BiOBr features a unique layered structure akin to PbFCl, consisting of alternating bismuth, oxygen, and bromine atoms Its properties and performance are significantly influenced by specific crystal facets, particularly (001) and (102) orientations The material possesses a relatively narrow bandgap, allowing it to effectively absorb visible light Notably, BiOBr demonstrates excellent chemical stability and low toxicity, making it suitable for environmental applications The layered architecture facilitates efficient charge separation, minimizing the recombination of electron-hole pairs, which is crucial for enhancing photocatalytic activity While its conductivity is moderate, it can be optimized through doping or the formation of composites.
Various synthetic methods for preparing BiOBr include hydrothermal, template-based, and sol-gel techniques This study focuses on synthesizing nanostructured BiOBr materials using a sol-gel approach combined with a hard template The sol is formed through the hydrolysis and condensation of bismuth and bromine precursors, which then gelates By introducing a soft template during this process, we aim to influence the nucleation and growth of BiOBr particles, thereby controlling the final morphology and properties of the resulting material.
The sol-gel technique is an effective method for preparing materials, particularly metal oxides, with controlled microstructures This wet chemical process transforms solutions containing metal precursors into colloidal particles through hydrolysis and condensation reactions, ultimately forming a gel-like network After drying and calcining the gel, the desired material is obtained For synthesizing BiOBr, common precursors like bismuth nitrate or bismuth oxide are dissolved in an appropriate solvent, followed by the introduction of catalysts.
11 that can promote hydrolysis The resultant sol is allowed to undergo controlled conditions leading to gelation and the formation of BiOBr particles[19]
Figure 2.3 Sol-gel method model
The hydrothermal method is widely used for synthesizing nanostructured materials like BiOBr and BiOCl This synthesis process involves several steps, beginning with the natural hydration of metal ions, followed by hydrolysis to create metal hydroxide precipitates Under high temperature and pressure, these precipitates dehydrate, leading to the nucleation and growth of nanostructures Importantly, the hydrothermal parameters significantly affect the reaction rate and the morphology of the final products, allowing for improved control over the properties of the synthesized materials.
Figure 2.4 Model of hydrothermal method
Introdution of nano Polystyrene
Polystyrene is a widely used plastic known for its low cost, lightweight nature, and durability, but its resistance to degradation poses significant environmental challenges Although some microorganisms can utilize polystyrene as a carbon source, its high molecular weight limits its direct use in many enzymatic processes In contrast to bulk material, polystyrene nanoparticles (NPS) have a reduced glass transition temperature and increased surface area, enhancing their versatility These properties, along with controllable size and adjustable surface characteristics, make NPS promising for various applications, such as carriers in biological systems and templates for nanomaterial synthesis, prompting ongoing research into their potential across diverse fields.
Polystyrene (PS) is a synthetic polymer created through the polymerization of styrene, also known as vinylbenzene Its structure features a linear hydrocarbon chain with phenyl groups attached to alternate carbon atoms, resulting in a non-crystalline, amorphous form held together by weak van der Waals forces However, the presence of hydrogen atoms bonded to tertiary carbon atoms within the polystyrene chains makes the material susceptible to oxidation, particularly when exposed to sunlight.
Figure 2.5 The structure of polystyrene
Spatial structural forms of PS are shown in Figure 2.6 In the atactic form, the phenyl groups are arranged randomly In the syndiotactic form, it is arranged in a regular
In isotactic polymers, functional groups are aligned on one side of the chain, while the syndiotactic configuration exhibits superior properties due to its symmetrical and orderly arrangement, leading to enhanced molecular bonding and improved mechanical and physical characteristics However, the synthesis of syndiotactic structures is complex and stringent, making them suitable primarily for high-technical specification applications.
Figure 2.6 Spatial structural configurations of polystyrene 2.3.3 Properties
Polystyrene (PS) displays distinct physical properties, characterized by low molecular weight, which leads to brittleness and reduced tensile strength due to bulky phenyl groups that restrict chain flexibility However, utilizing high molecular weight can enhance its mechanical and thermal properties PS is effective within a temperature range from ambient conditions up to approximately 80°C, with a burning point of 173°C and decomposition occurring around 330°C Notably, PS has a broad processing window, enabling versatile shaping of products.
Table 2.1 Physical properties of PS
Melting entropy (KJ/mol) 0.0153 – 0.0168 Heat capacity (100K) (KJ/mol) 15.6 – 21.1
Polystyrene, typically white or transparent, is a non-biodegradable and relatively nontoxic material, although its safety can be compromised by additives or solvents It possesses waterproof properties and acts as an insulator due to its non-polar nature While polystyrene remains stable in polar solvents like dilute cleaning solutions, it is less stable when exposed to non-polar solvents such as aromatic hydrocarbons, toluene, and chloroform.
Polystyrene, a rigid thermoplastic, exhibits specific mechanical properties detailed in Table 2.2 Its specific gravity can be adjusted through blending with other polymers to tailor its characteristics for various applications This material maintains a continuous temperature service range of approximately 59 to 79°C; however, prolonged exposure to high temperatures can degrade its properties.
Table 2.2 Mechanical properties of polystyrene
Mass polymerization is a method where the liquid monomer serves as the medium, with the initiator dissolved directly in it This process may include molecular weight modifiers that are soluble in the monomers, avoiding the use of solvents However, the high viscosity created can lead to poor temperature distribution and agglomeration of active centers, resulting in non-uniform polymerization rates and reduced process efficiency Additionally, the final product typically requires the removal of unreacted monomers Despite these drawbacks, the simplicity of mass polymerization is a significant advantage, as the removal of unreacted monomers yields a highly purified product.
Solution polymerization is a method involving the dissolution of monomers and initiators in solvents, allowing for enhanced control over temperature and viscosity However, this technique presents significant challenges, including the difficulty of solvent removal from the final product and the toxicity of many solvents, which can lead to environmental pollution Additionally, solution polymerization may result in lower molecular weight products due to issues related to circuit transmission processes.
Suspension polymerization involves dispersing monomers in water to create droplets, where initiators dissolve these monomers, resulting in water-based polymer suspensions that can be easily separated through filtration and washing This method utilizes water for efficient heat transfer, maintaining low solution viscosity For optimal results, the polymer's glass transition temperature (Tg) must be lower than the polymerization temperature; a high Tg can lead to increased flexibility, agglomeration, and reduced stability of the polymer.
This technique involves using an emulsion of monomer dispersion stabilized by surfactants, such as organic fatty acids, which reduce water's surface tension to facilitate monomer dispersion These surfactants exhibit low solubility, allowing them to diffuse in aqueous media even at low concentrations When the concentration reaches a certain level, insoluble surfactants aggregate to form micelles The critical micelle concentration (CMC) is the point at which all surfactant molecules are dispersed and micelles are formed, characterized by a structure with a hydrophilic outer surface and a hydrophobic core.
When an initiator is introduced into the system, polymerization starts at the micelle's surface and progresses inward Throughout this process, monomers are consumed while continuously diffusing from the water phase into the micelles Polymerization continues until a free radical emerges, which halts the reaction The resulting polymer accumulates within the micelles, remaining suspended in water due to the stabilizing effect of the emulsifier Emulsion polymerization not only lowers the solution's viscosity for direct application but also complicates heat transfer because of the polymer's low viscosity.
2.3.4.5 Quasi-emulsifier-free emulsion polymerization
Emulsion polymerization is inherently complex, making it challenging to control the reaction process, which often leads to a non-uniform dispersion of polymer particle sizes High surfactant concentrations can extend nucleation time and create multiple dispersions In contrast, emulsifier-free emulsion polymerization minimizes or eliminates surfactants, resulting in more uniform nucleation and consistent polymer particle sizes.
During the reaction, oligomers form and ultimately create micelles, where monomers interact to produce polymers In the absence of surfactants, these particles are significantly less stable and tend to break apart, leading to the formation of larger particles Therefore, synthesizing through emulsification with a surfactant concentration below its critical micelle concentration prevents secondary nucleation and ensures the stability of micelles formed by oligomers.
Method of fabricating nanostructures with hard templates
Hard templates create a reaction cavity through covalent bonds, facilitating the diffusion of substances in and out of the cavity walls The preparation of materials using hard templates involves several steps, beginning with the coating or infiltration of a chosen template with a precursor material This precursor is solidified through chemical or thermal treatment, and once hardened, the hard template is removed via chemical etching or high-temperature calcination The interaction between the template molecules and the precursor results in pore channels within the final porous material, which remain after the template's removal.
Figure 2.7 Illustrating the Hard template method
EXPERIMENTALS
Materials
Table 3.1 Chemicals used in the project
No Material’name Details Source
Bismuth III nitrate pentahydrate (Bi(NO3)3.5H2O)
(CH3CH2OH) 1000 mL China
Apparatuses and equipment
Table 3.2 Apparatuses used in the project
Three – neck round – bottom flask 250 mL
Table 3.3 Equipment utilized in the project
No Appliance’s name Model Image
Field emission scanning electron microscope
6 Fourier-transform infrared spectrometer iS50 Nicolet
8 UV-Vis DRS spectrophotometer Jasco - 770
Fabrication process
NPS was synthesized through quasi-emulsifier-free emulsion polymerization, as illustrated in Figure 3.1 In a three-necked flask under a nitrogen atmosphere, 4 mL of styrene monomer (MS) and 3 mmol/mL of sodium dodecyl sulfate (SDS) were dissolved in distilled water and stirred at 300 rpm for 30 minutes to achieve a uniform dispersion The mixture was then heated to 70 °C, at which point 3.69 mmol/mL of potassium persulfate (KPS) was introduced as a polymerization initiator This initiated the polymerization reaction, which continued for 30 minutes, resulting in the formation of the latex product.
Figure 3.2 Nano Polystyrene synthesis process experiment
BiOBr synthesis process
Figure 3.3 Diagram of synthesis of BiOBr by hard teamplate method
BiOBr was synthesized using the sol-gel method, starting with the dissolution of 1.28g of Bi(NO3)3·5H2O in a mixed solvent of ethylene glycol (EG) and water, along with 1.0 mol/L of Tu at room temperature Following the formation of a white precipitate, 0.27g of KBr was added, and the reaction continued for 60 minutes The resulting product was then filtered, washed with distilled water and ethanol, and dried at 40°C for 6 hours to yield the final BiOBr product.
NPS-based BiOBr synthesis process
Figure 3.4 NPS-based BiOBr synthesis process
“The synthesis process of BiOBr by NPS-based hard template method is displayed in Figure 3.4 Firstly, the precursor Bi(NO3)3ã5H2O was dissolved in EG and
In this study, a mixed H2O solution was prepared using the hard-template method, followed by the addition of KBr, which reacted for 30 minutes to produce a white precipitate Subsequently, varying concentrations of NPS (10wt%, 30wt%, 50wt%, 70wt%, and 90wt%) were introduced into the system, allowing for an additional 60-minute reaction The final product was obtained through centrifugation and multiple washings with CHCl3.
Figure 3.5 Experimental synthesis of BiOBr by sol gel and hard template method
Table 3.4 Samples were synthesized according to the proposed procedures
No Sample’name Amount of BiOBr
Characterization
❖ Fourier Transform Infrared Spectroscopy (FTIR)
Fourier Transform Infrared Spectroscopy (FTIR) is a rapid analytical technique that converts the interferogram from an interferometer into an infrared spectrum using Fourier transformation This method allows for both qualitative and quantitative analysis of inorganic and organic materials in solid, liquid, and gaseous states without altering their composition The results are presented as percentage transmittance against wavelength (cm⁻¹) FTIR operates on the principle that selective absorption of infrared radiation by a molecule alters its dipole moment, resulting in frequency variations of molecular bond oscillations, which correspond to specific energy levels defined by the equation E = hf, where h is the Planck constant and f is the frequency.
Infrared absorption occurs at specific wavelengths within the infrared spectrum, indicating the presence of distinct chemical bonds The intensity of these absorption bands is directly proportional to the concentration of the respective bonds in a sample Infrared absorption bands are categorized into three regions: near infrared, mid-infrared, and far-infrared.
The bonds present in the synthesized material were characterized in this study using the middle infrared region within the range of 4000–400 cm -1
FT-IR analysis identifies the vibrational modes of bonds, allowing for the determination of their presence in materials This investigation utilized the Nicolet iS50 FTIR Spectrometer from the University of Finance and Marketing.
❖ Scanning Field Emission Microscopy (FE-SEM)
Scanning Electron Microscopy (SEM) utilizes high-energy electrons rather than visible light to achieve high-resolution imaging By accelerating electrons within a voltage range of 10 kV to 40 kV, SEM enables detailed interaction with the sample's surface, revealing intricate features and dimensions at the microscopic level.
Scanning Electron Microscopy (SEM) operates by utilizing the interaction between primary electrons and a sample, leading to the generation of secondary electrons When these primary electrons collide with the sample surface, they impart energy to surface atoms, causing ionization and the ejection of secondary electrons These secondary electrons are collected by the detector, providing crucial insights into the elemental composition of the sample's surface Additionally, primary electrons can also be backscattered upon encountering atoms in the sample, with the degree of scattering influenced by the atomic number of the involved atoms, ultimately affecting the intensity of the detected backscattered electrons.
Scanning Electron Microscopy (SEM) functions in a vacuum, allowing electrons to travel freely from the source to the sample, which minimizes interference and enhances image quality By employing a field emission gun, SEM can produce shorter wavelengths of electrons, resulting in higher resolution images This advanced technique is referred to as field emission scanning electron microscopy.
This study employs short-wavelength electrons to deliver high-resolution insights into the surface characteristics of materials The research utilizes the Hitachi S-4800 Field Emission Scanning Electron Microscope (FE-SEM) located at the Saigon High-Tech Park (SHTP) Laboratory.
Dynamic Light Scattering (DLS), also known as Photon Correlation Spectroscopy, is a key technique for analyzing the morphology of nanoparticles in colloidal dispersions This method measures fluctuations in light scattering intensity caused by the Brownian motion of nanoparticles, where larger particles exhibit slower movement compared to smaller ones, influencing the scattering intensity The resulting intensity fluctuations are analyzed using computational methods to derive a time correlation function, enabling detailed interpretation of nanoparticle behavior.
The radius and size of particles in a system at nano and micro scales can be determined by correlating the diffusion coefficient with particle size through the Stokes– Einstein equation:
KB is the Boltzmann constant, η is the viscosity of the system, T is the absolute temperature and R is the particle radius[29]
Particle size distribution at the nanoscale scale is provided via DLS analysis The Institute of Nanotechnology Horiba LB550 particle size analyzer was used in this investigation
X-ray diffraction (XRD) can be applied in the structural examination of a sample without resulting in damage It's used to determine details on the size, crystalline properties, purity, and quantity of solid samples This is achieved by passing monochromatic X-rays obtained by heating a filament in a cathode ray tube toward the crystal The X-rays diffract as they interact with the crystal lattice, resulting in a pattern of intensity peaks and troughs The X-ray source is fixed at an angle θ at the sample, and the detector rotates at an angle of 2θ The detector records the intensity of the diffracted X-rays as the sample is rotated The diffraction patterns recorded are then analyzed in the course of data processing for the final X-ray diffraction spectrum[30]
X-ray diffraction studies reveal the structure and crystal size of the material This research utilizes the Malvern Panalytical Empyrean Series 3 multipurpose X-ray diffractometer from the University of Finance and Marketing.
For a diffraction peak to form, the angle of incidence of X-rays must adhere to Bragg's law, given by the equation:
2𝑑 × sin 𝜃 = 𝑛𝜆 where d represents the distance between crystal planes, and 𝑛 n is an integer value
The size of the crystal is determined through the Scherrer equation:
D is the crystal size (nm), λ is the wavelength of X-rays (nm), β is half the width of the diffraction peak (FWHM) (rad), θ is the Bragg angle (rad),
K is the Scherrer constant, they vary from 0.8 – 1.39 which depends on the crystal shape and the index of the diffraction line[30]
UV-Vis spectroscopy is an analytical method that quantifies the absorption or transmission of UV and visible light by a sample compared to a reference This technique reveals information about the sample's composition and concentration The energy of light is inversely related to its wavelength, meaning shorter wavelengths possess higher energy, which is essential for promoting electrons to higher energy states detectable as absorption Different bonding environments in a substance require varying energy amounts for electron excitation, leading to distinct light absorption at different wavelengths.
Humans can perceive a spectrum of visible light ranging from approximately 380 nm (violet) to 780 nm (red), while ultraviolet (UV) light has wavelengths shorter than visible light, down to about 100 nm In UV-Vis spectroscopy, light is characterized by its wavelength, allowing for the analysis and identification of various substances by identifying specific wavelengths that correspond to maximum absorbance The relationship between absorption intensity and solution concentration follows the Lambert-Beer law.
Where: A is absorption, Ɛ is molar absorption, b sample thickness, c is concentration, T is transmittance
The material composition of a sample can be effectively analyzed through UV-Vis spectroscopy This study utilized the Jasco V-730 UV-Vis spectrophotometer at Ho Chi Minh City University of Technology and Education.
Visible ultraviolet diffuse reflectance spectroscopy is an optical technique used to characterize catalysts and semiconductor materials by determining absorption peaks or band gaps This method involves directing light onto a material, resulting in both specular and diffuse reflections at the surface, with the diffuse reflectance spectrum providing insights into the electronic structure and other material properties The band gap (Eg) of a material can be calculated using the Tauc equation, which establishes a relationship between Eg and absorption.
In the Tauc equation, α represents the absorption coefficient, ℎ𝑣 denotes the photon energy, 𝑛 is a parameter that indicates whether the material is a direct or indirect semiconductor, and C1 is a proportionality constant
The absorption capacity of the material is illustrated in the DRS spectrum, which is essential for calculating the sample's band energy value This study utilizes the Jasco V-770 UV-Vis spectrophotometer from Ho Chi Minh City University of Technology.
Photocatalytic survey process
To produce the solution, a calibration curve was calculated using the equation Ax
The relationship between the known concentration of Rhodamine B (RhB) and the sample's absorbance is represented by the equation f(Cx) A standard curve was created and analyzed using linear regression to accurately determine the concentration of RhB in milligrams per liter (mg/L).
During the benchmarking process, we adjusted the color concentration by modifying the ratio of RhB content Initially, we prepared a stock solution with a concentration of 1000 mg/L, from which we created subsequent solutions with concentrations ranging from 15 mg/L to 1 mg/L.
UV-Vis spectroscopy was used to locate the absorption peaks in the diluted solutions Plot the absorbance (A) against the concentration (C) graph to determine the concentration Cx
Table 3.5 Data used to construct the standard curve for RhB
Figure 3.7 Calibration curve plotted in origin
The graph in Figure 3.7 shows a correlation coefficient of r² = 0.99, indicating a high level of accuracy in determining Cx using the linear regression equation
3.5.2 Adsorption and photocatalytic activity measurements
The concentration was 10 mg/L using a RhB of 100 mL and BiOBr photocatalyst in the form of material amounting to 25 mg for this study You can see the model in
In a controlled experiment using a 50 W LED light and a magnetic stirrer, samples were collected every 5 minutes after a 30-minute absorption equilibration period to monitor the concentration changes of Rhodamine B (RhB) over time.
Figure 3.8 A photochemical model for RhB degradation
RESULTS AND DISCUSSIONS
Characterization of as-prepared NPS
Researchers employ FTIR spectroscopy to investigate bond vibrations, X-ray diffraction to assess crystal structure, DLS analysis for particle size distribution, and field emission scanning electron microscopy to examine surface morphology in their study of nano polystyrene synthesized via quasi-emulsifier-free emulsion polymerization.
4.1.1 Fourier Transform Infrared Spectroscopy (FTIR)
Figure 4.1 FTIR spectrum of Nano Polystyrene (NPS)
In FTIR spectroscopy, the bonds found in the materials were represented by their vibrations, as illustrated in Figure 4.1 The FTIR spectrum showed that the peaks at
The absorption peaks observed at 3084 cm -1 and 3025 cm -1 correspond to the vibrations of the C-H bond in the benzene ring, while the peaks at 2926 cm -1 and 2850 cm -1 are linked to the stretching vibrations of the same bond Additionally, the prominent absorption peaks at 1600 cm -1, 1495 cm -1, and 1452 cm -1 represent the vibrations of C=C bonds in aromatic hydrocarbons The bending vibrations of the C-H group in the benzene ring are indicated by peaks at 756 cm -1 and 696 cm -1 These findings align with previous analyses of the bond vibrations in the NPS sample.
Figure 4.2 X-ray pattern of NSP
X-ray diffraction (XRD) spectroscopy enabled the analysis the structural morphology of a material The results of the X-ray diffraction analysis conducted in a 2 theta range from 5° beyond, were presented in Figure 4.2 for every sample that was examined Polymeric materials have a somewhat bigger peak in their diffusion pattern at average distances between a few layers due to the mutual contribution of individual plane diffractions The groups that are relatively crystalline inside polymeric materials and the peaks at 19.16° occur in sections with an angle range of around 9.23° The results of past research on PS materials are consistent with these diffraction patterns[34]
From XRD and FTIR spectroscopy, NPS was synthesized effectively, exhibiting X-ray diffraction peaks and distinctive oscillations in the material
Figure 4.3 Nano Polystyrene (NPS) particle size distribution graph
The particle size distribution of nanoparticles (NPS) was analyzed through Dynamic Light Scattering (DLS), revealing that most NPS particles are under 100 nm in size, with a significant portion concentrated within a specific range.
The particle size distribution of the nanoparticles (NPS) predominantly ranges from 60 to 90 nm, with 76 nm being the most common size This finding indicates that the emulsion polymerization method, utilizing minimal emulsion, successfully controls the particle size distribution.
Figure 4.4 SEM images of NPS are shown at (a) 100k magnification and (b) 200k magnification
Scanning Field Emission Electron Microscopy (SEM) was utilized to assess the surface morphology of nanoparticles (NPS), revealing uniformly distributed particles that exhibited round spherical shapes with smooth surfaces and minimal surface features The magnifications used ranged from 100K to 200K, and the particles were found to have interparticle intervals that were closely spaced, measuring between 70–76 nm, consistent with the Dynamic Light Scattering (DLS) data presented.
The DLS and SEM analyses demonstrated that the NPS was synthesized as a monodisperse material with particle sizes under 100 nm Further evaluations, including X-ray diffraction and SEM surface analysis, confirmed the high purity of the NPS, indicating it was free from contaminants.
Characterization of mesoporous BiOBr
The sol-gel process and hard template method utilizing NPS are employed for the synthesis of BiOBr To analyze its surface morphology and structural characteristics, techniques such as FTIR, XRD, and FE-SEM are utilized Additionally, UV-Vis and UV-Vis DRS spectroscopy are conducted to evaluate the photochemical properties and performance of the synthesized materials.
4.2.1 (FTIR) Fourier Transform Infrared Spectroscopy
Figure 4.5 FTIR diagram of BiOBr synthesized by sol-gel method and
PS,PS10,30,50.70.90 synthesized by hard template method
The FTIR spectrum indicates the presence of various functional groups in BiOBr, PS, and other samples The absorption peak of pure BiOBr at 520 cm−1 corresponds to the stretching vibrations of the Bi-O bonds A notable shift occurs from the original peak at 3375 cm−1, related to hydroxyl group vibrations, to a lower wavenumber of 3366 cm−1 Additionally, the in-plane -OH deformation vibration is observed at 1740 cm−1, while O-H bond vibrations peak at 3430 and 1620 cm−1, suggesting moisture absorption during storage Chemically fabricated composites show a significant reduction in the Bi-O peak, similar to that of PS70, likely due to the incorporation of NPS The Bi-Br bands appear as broad peaks in the 1000 to 1500 cm−1 range, with residual ethylene glycol identified by an absorption peak nearby.
Figure 4.6 X-ray patterns of BiOBr and PS70
The XRD method was utilized to analyze the crystal phase structure of the prepared samples, as illustrated in Figure 4.6 The diffraction patterns of the two samples showed similar characteristics In the case of pure BiOBr, distinct diffraction peaks were observed at angles of 10.8°, 25.2°, 31.7°, 32.3°, 46.3°, and 57.2°, corresponding to the crystal planes (001), (101), (102), and (110).
The analysis of BiOBr revealed clear and sharp peak intensities at 200 and 212, aligning well with the JCPDS file card 09-0393 The absence of additional peaks indicates high purity and excellent crystallization of the product, corroborating previous research findings.
The successful synthesis of the BiOBr sample was confirmed through FTIR and X-ray diffraction measurements, which revealed unique oscillations and diffraction peaks indicative of the material's properties Additionally, the absence of polystyrene bonds and diffraction peaks in the PS70 sample demonstrated the effective removal of the template, resulting in a pure and highly crystalline product.
Figure 4.7 Side 001 of BiOBr and PS70 magnified
The crystal size was determined using the Scherrer equation based on the diffraction peaks presented in Figure 4.7 Notably, the peak width for PS70 on the (001) plane increased, with similar broadening observed in the (002), (101), (110), (102), (112), and (212) planes This expansion of the lattice planes resulted in a reduction in crystal size, as detailed in Table 4.1 These findings suggest that BiOBr synthesized via the hard template method yields a smaller crystal structure compared to that produced by the sol-gel method.
Table 4.1 Sample crystal size BiOBr and PS70
Sample’s name FWHM( o ) 2θ( o ) Crystal size (nm)
Figure 4.8 Surface morphology of samples a) BiOBr sample at 50K magnification,b)
BiOBr sample at 100K magnification, c) PS70 sample at 50K magnification, d) PS70 sample at 100K magnification
The morphology and size of the samples were examined using SEM, as illustrated in Figure 4.8 The SEM images in Figure 4.8a reveal that both pure BiOBr and PS70 exhibit a flower-like structure, measuring approximately 1.7 - 2 μm, composed of aggregated BiOBr sheets These sheets have an average thickness of 20 - 25 nm and a size of about 100 nm In contrast, the PS70 sample shown in Figure 4.8d features a rougher surface compared to pure BiOBr, with more uniform layer sizes and BiOBr sheets that are tightly interwoven, measuring 10-15 nm thinner than those in the pure BiOBr sample.
The PS70 structure, observed at a magnification of 100k, reveals distinct concave regions, highlighting the significant porosity that enhances surface properties compared to pure BiOBr This suggests that the presence of NPS plays a crucial role in the structure and morphology of the semiconductor SEM results indicate that NPS substantially increases the specific surface area of BiOBr due to the numerous porosities and pores within the material, attributed to the small size of NPS, which is less than 100 nm in diameter.
44 far smaller than BiOBr molecules themselves and can thus be incorporated into mesoporous structures
Figure 4.9 UV-Vis DRS spectrum of BiOBr and PS70
The DRS spectrum results, illustrated in Figure 4.9, reveal the optical properties of the prepared samples, with BiOBr and PS70 exhibiting absorption edges at approximately 470 nm and 490 nm, respectively, highlighting their capability to absorb visible light, especially blue light Notably, the absorption edges of the PS70 samples shift towards higher wavelengths, likely due to interactions between BiOBr and NPS or alterations in BiOBr's crystal structure in the presence of NPS Ultimately, NPS enhances the material's light absorption capacity by increasing the specific surface area, facilitating improved light absorption and more efficient solar energy utilization.
Figure 4.10 The band gap energy value of BiOBr, PS70
The band gap energy was calculated using the Tauc equation, as outlined in section 2.4.7 The results indicated a band gap energy of 2.73 eV for BiOBr and 2.64 eV for the PS70 model, as illustrated in Figure 4.10.
The presence of NPS resulted in a slight decrease in the band gap energy, from 2.73 eV for BiOBr to 2.64 eV for PS70, suggesting that quantum effects may play a role This reduction in band gap energy indicates that the surface area influences the band gap value, as the concentration of oxygen vacancies increases with surface area, leading to a narrower band gap.
Table 4.2 The band gap energy value of BiOBr, PS70
Photocatalytic Results
RhB adsorption at room temperature for 30 minutes in the dark adheres to the pseudo-second-order kinetic model The adsorption capacity of RhB, along with the corresponding rate constant, can be calculated using a specific equation.
In this study, qt (mg/g) indicates the quantity of RhB adsorbed onto the photocatalyst at a specific time t, while qe (mg/g) represents the equilibrium amount of RhB adsorbed Additionally, kasd (g/mg.min) denotes the adsorption rate constant.
Figure 4.11 Kinetic data of Rhb adsorption in the dark and (B) corresponding pseudo- second-order kinetic plots
Figure 4.11 illustrates the varying values of qe and kads for the adsorption of RhB, with detailed results summarized in Table 4.3 The qe values, which range from 7.73 to 25.41 mg/gcat, indicate a ranking of adsorption efficiency as follows: PS70 > PS90 > PS50 > PS30 > BiOBr.
The analytical results were compatible with the surface observed in the previous sections The porous structure formed by NPS particles, which were smaller than 100
47 nm, greatly enhanced both the volume and surface area of the material This improvement led to better adsorption of RhB and enhanced photochemical properties of BiOBr
Table 4.3 Kinetic data for rhb adsorption
Sample q e (mg/g cat ) K ads (g/mg.min) r 2
Figure 4.12 Photocatalytic activity graph for samples with RhB concentration of 10 mg/L
The study revealed a notable reduction in RhB dye concentration, with the PS10 sample achieving approximately 70% degradation, which was less effective than the pure BiOBr sample This difference may stem from potential errors in sample preparation or measurement In contrast, the PS70 and PS90 samples exhibited superior performance, nearly fully degrading the RhB dye within 25 minutes of catalysis This improvement highlights that the incorporation of NPS significantly enhanced the adsorption capacity of the samples As the NPS content increased from 30% wt to 90% wt, both the surface area and porosity of the material improved, thereby boosting its adsorption capabilities.
The illumination data is in Figure 4.12 Revealed that all samples achieved a
The study demonstrated a remarkable reduction in RhB, exceeding 98% across most samples, with the PS10 sample showing a 70% reduction The degradation rates for the various samples were notably high: BiOBr achieved a 98.35% reduction, PS30 reached 98.7%, PS50 recorded 99.3%, PS70 attained 99.8%, and PS90 exhibited a 99.5% reduction.
Figure 4.13 Graph of the decline in Rhb concentration over time of the sample
The reaction rate constant of BiOBr is expressed under the first-order kinetic reaction model:
Ln(Co/Ct) = kapp t kapp is the reaction rate constant,
Ct is the Rhb concentration at time t,
Co is the initial Rhb concentration
The reaction rate constant (k) can be determined by plotting Ln(Co/Ct), where kapp represents this constant The values obtained are depicted in Figure 4.13, corresponding to the BiOBr, PS10, PS30, PS50, PS70, and PS90 models, with detailed calculations provided in the accompanying Table.
Table 4.4 The value of the reaction rate constant of the samples
Sample’name Efficiency (%) k app (min -1 ) R 2
The analysis of the reaction rate constants in Table 4.4 reveals that the samples ranged from 0.08 to 0.15 min⁻¹ BiOBr exhibited the lowest reaction rate constant at 0.08 min⁻¹, whereas PS70 demonstrated the highest at 0.19 min⁻¹ This suggests that the effective visible light absorption in the PS70 structure leads to a reaction rate that is 2.2 times faster than that of BiOBr.
The analysis of DRS and UV-Vis spectra indicates that NPS significantly enhances the photoreactivity of BiOBr by increasing its specific surface area This enhancement leads to improved RhB adsorption capacity, ultimately resulting in a higher rate of photocatalytic activity.
Figure 4.14 Graph of time on the ability of PS70 to decrease Rhb concentration
The degradation of RhB using sample PS70 was assessed over 60 minutes, with results analyzed through UV-Vis spectroscopy The characteristic absorption peak of RhB at approximately 553 nm decreased significantly within the first 5 minutes of illumination, reaching near-zero levels by 15 minutes, demonstrating PS70's strong efficacy under visible light Notably, a peak shift occurred during the photochemical reaction, likely due to the formation of byproducts Ultimately, both RhB and its byproducts were fully converted into CO2 and H2O by the end of the process.
In conclusion, the thesis successfully accomplished its primary objectives:
The study successfully synthesized monodispersed nanoparticles (NPS) under 100 nm, which served as effective hard templates Furthermore, BiOBr was synthesized using this hard template method, with the removal of the templates verified through FT-IR, FE-SEM, and XRD analyses.
The study demonstrated that integrating NPS substantially improved both the pore size and specific surface area of the material This enhancement in specific surface area directly influenced photocatalytic efficiency, as a larger surface area is associated with an increased capacity to decompose harmful organic compounds.
The thesis still has limitations due to human factors and time constraints, leaving room for further development Here are some suggestions for future research:
Explore additional materials that can be combined with BiOBr to enhance its photocatalytic properties Use other solvents to remove the template such as toluene, acetone
Investigate the degradation of other organic compounds beyond RhB, such as
MB, MO, CR… and heavy metals
Experiment with alternative solvents like toluene or acetone to remove PS the template
Study the recovery and reuse of the material after the completion of the photocatalytic reaction
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Researchers Nguyen Hong Hanh, Quan Thi Minh Nguyet, Tran Van Chinh, La Duc Duong, Tran Xuan Tien, Lai Van Duy, and Nguyen Duc Hoa have published a study in RSC Advances, Volume 14, Issue 21, on May 2, 2024 The article focuses on the enhanced photocatalytic efficiency of porous ZnO coral-like nanoplates for the degradation of organic dyes The findings highlight the potential of these nanoplates in environmental applications, particularly in wastewater treatment For further details, refer to the publication with DOI: 10.1039/D4RA01345J.